Successful user operation of a superconducting radio-frequencyphotoelectron gun with Mg cathodes

J Teichert 1 A Arnold 1 G Ciovati2 J-C Deinert 1 P Evtushenko 1 M Justus1

J M Klopf 1 P Kneisel2 S Kovalev 1 M Kuntzsch 1 U Lehnert1 P Lu1dagger S Ma 1

P Murcek1 P Michel 1 A Ryzhov 1 J Schaber 13 C Schneider1 R Schurig1

R Steinbruumlck1 H Vennekate 1Dagger I Will4 and R Xiang 1

1Helmholtz-Zentrum Dresden-Rossendorf 01328 Dresden Germany2Thomas Jefferson National Accelerator Facility Newport News Virginia 23606 USA

3TU Dresden 01062 Dresden Germany4Max-Born-Institut 12489 Berlin Germany

(Received 14 November 2020 accepted 1 February 2021 published 4 March 2021)

At the electron linac for beams with high brilliance and low emittance (ELBE) center for high-powerradiation sources the second version of a superconducting radio-frequency (SRF) photoinjector has beenput into operation and has been routinely applied for user operation at the ELBE electron accelerator SRFguns are suitable for generating a continuous wave electron beam with high average currents and high beambrightness The SRF gun at ELBE has the goal to generate short electron pulses with bunch charges of200ndash300 pC at typical repetition rates of 100 kHz for the production of superradiant coherent terahertzradiation The SRF gun includes a 35-cell 13-GHz niobium cavity and a superconducting solenoidA support system with liquid nitrogen (LN2) cooling allows the operation of normal-conducting highquantum efficiency photocathodes We present the design and performance of the SRF gun as well as beammeasurement results of the operation with Mg photocathodes at an acceleration gradient of 8 MV=m(4 MeV kinetic energy) In the last section we discuss the SRF gun application for production of coherentterahertz radiation at the ELBE facility

DOI 101103PhysRevAccelBeams24033401

I INTRODUCTION

The ldquoelectron linac for beams with high brilliance andlow emittance (ELBE)rdquo is a user facility open forinternational research projects with electrons and secondaryradiation Located at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) the ELBE accelerator with its super-conducting linac has the unique feature to operate incontinuous wave (cw) mode for high average power output[1] And quite obviously a reliable cw electron source isrequired for this There are various candidates of such cwsources for instance the state-of-the-art dc guns [23] orthe conceptual relatively new superconducting radio-frequency (SRF) guns [4] as well as low-frequency normalconducting guns [5] For the ELBE facility a thermionic dcgun of 235 kV routinely provides up to 77 pC at 13 MHz

for the high-power IR free-electron lasers (FELs) and otherusers However high bunch charge operation is highlydemanded by several new attractive applications Forexample neutron time-of-flight experiments [6] requirehigh bunch charge to improve the signal-to-noise ratioHigh-power terahertz experiments [7] and Thomson back-scattering experiments [8] need high bunch charge andshort electron pulses to increase the intensity of thesecondary radiationAs part of the RampD effort to explore a new cw injector

for the ELBE center the first SRF gun was developed andoperated from 2007 to 2014 as the second injector [9] Thisgun consisted of a 13-GHz 35-cell niobium cavity achoke filter with a normal conducting photocathode a drivelaser a beam transport system and all other accessories InApril 2013 first lasing of one of the IR FELs at ELBE wasdemonstrated using the electron beam from this gun [10] Itrepresented an excellent demonstration that L-band SRFguns can work in a user facility but significant improve-ment was needed for better gun performanceBased on the experience of the first gun an updated

version has been installed in the accelerator hall since May2014 This so-called SRF gun II is now able to achieve anacceleration gradient of 8 MV=m which corresponds to205 MV=m peak field on axis In combination with Mg

jteicherthzdrdedaggerPresent address KLA-Tencor Shanghai ChinaDaggerPresent address TJNAF Newport News Virginia

Published by the American Physical Society under the terms ofthe Creative Commons Attribution 40 International licenseFurther distribution of this work must maintain attribution tothe author(s) and the published articlersquos title journal citationand DOI

PHYSICAL REVIEW ACCELERATORS AND BEAMS 24 033401 (2021)Editors Suggestion

2469-9888=21=24(3)=033401(16) 033401-1 Published by the American Physical Society

cathodes and a 258-nm laser this new gun routinelyprovides electron beams with bunch charge of up to300 pC This value is limited by the space charge effecton cathode surface In past years further improvements andoptimization of components as well as operational expe-riences have increased the performance and reliability sothat SRF gun II is now applied for user operation In thispaper we introduce the main features of the gun and sharethe experience gained during the commissioning andoperation for high-power terahertz user beam applicationThe advantages of the ELBE SRF gun II are the high

field gradient the high duty factor beam operation due to itssuperconducting technology and the good vacuum con-ditions suitable for sensitive photocathodes Depending onthe photocathode and the drive laser system the repetitionrate can be theoretically as high as gigahertz level Thusthe concept of SRF guns is promising to achieve highgradient and high output power [11] SRF guns are alsosuitable to be used in other applications such as energyrecovery linac projects [12] high duty factor x-ray FELs[13] and MeV ultrafast electron diffraction and micros-copy [14]

II OVERVIEW OF SRF GUN II DESIGN

The design concept of SRF gun II is similar to that of theprevious gun version which is described in detail elsewhere[9] But the cryomodule including cavity and supercon-ducting solenoid has been rebuilt completely In 2014 theold gun was replaced whereas the cathode transfer systemelectron beam lines and diagnostics were refurbished andjust slightly improved Over the years major changes weremade at the UV drive laser and the fundamental powerradio-frequency (rf) system A summary of the main gunparameters is given in Table I The following detaileddiscussions are focused on design issues that are not

discussed in previous papers and on subsystems that differsignificantly in SRF gun II

A Cryomodule design cavity SC solenoidand cathode cooling

The cryomodule is designed for cw operation of asuperconducting Nb cavity cooled down to 2 K For thispurpose the cavity is housed in a titanium vessel filled withsuperfluid helium using a two-phase helium pipe that at thesame time collects the evaporated gas to return it to thehelium plant The maximum heat load into the helium bathhandled by this pipe can be as high as 40W (or 2 g=s) Thisvalue is composed of 7 W static loss caused by the finitethermal isolation and up to 33 W dynamic loss Since thelatter is determined by the field inside the cavity an electricheater is used to operate the cryomodule at constant heliumheat load (or mass flow) regardless of the cavity gradientThe heater is placed in a small helium pot that serves as anadditional helium reservoir and guarantees a constant filllevel in the cryomodule This has been proven advanta-geous to achieve a pressure stability of down to 01 mbarrms in the whole helium system A computer-aided design(CAD) layout of the cryomodule is shown in Fig 1 Thestainless steel vacuum vessel has a cylindrical shape with07 m diameter and 13 m length Both the He port and theLN2 port are on top of the cryomodule The cryomodule isequipped with a liquid-nitrogen-cooled thermal shieldoperated at 77 K 10ndash20 layers of superinsulation foiland a warm magnetic shield (close to room temperature)made of MUMETALLregFor the high bunch charge operation the magnetic lens

was moved inside the cryomodule and is now located atabout 07 m from the cathode In contrast to SRF gun Iwhere a normal conducting solenoid was placed outside themodule at about 11 m the new superconducting solenoidenables the compensation of space charge force earlier andthus improves the transverse emittance [15]As shown in Fig 1 the 13-GHz cavity consists of three

cells with shapes like the design for the TeV-EnergySuperconducting Linear Accelerator (TESLA) and alength-optimized half-cell A fifth superconducting cellcalled the choke filter is surrounding the cathode area andprevents rf leakage into the cathode support systemCompared to SRF gun I the half-cell is improved by astiffening structure to reduce Lorentz force detuning [16]Furthermore the half-cell is now tuned to 80 of the on-axis peak field of the TESLA cells of the cavity whereasthis value was 60 in SRF gun I This is possible by apermanent pretension at the half-cell tuner of the double-tuning system of the gun [9] Together with the higheracceleration gradient of SRF gun II it doubles the electricpeak field at the cathode to 145 MV=m with respect toSRF gun I This value is true if the cathode is retracted by13 mm together with a bias of minus5 kV as it turned out to bethe optimum case for operation with Mg photocathodes and

TABLE I SRF gun II main parameters

Value

Parameter Gun I Gun II

Type of cavity Elliptical 35 cellsFrequency 13 GHzGun operation mode cwBeam energy at gun exit 30 MeV 40 MeVAcceleration gradient Eacc 6 MV=m 8 MV=mPeak field on axis 162 MV=m 205 MV=mCathode field 7 MV=m 145 MV=mdc bias at cathode minus5 kVLiquid He temperature 2 KDynamic He load at max Eacc 10 WDrive laser wave length 258 nmPhotocathodes Cs2Te MgQuantum efficiency 1 01ndash03

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higher bunch charge (200 pC) required for user operationat ELBEWithin a collaboration project the SRF gun II cavity was

fabricated treated and tested at the Thomas JeffersonNational Accelerator Facility (TJNAF) [17] The cavity wasvertically measured after final treatment at TJNAF in 2013and later in the gun cryomodule with a copper cathodeduring the commission process in 2014 Since then theperformance of cavity has been monitored regularly Someselected results are shown in Fig 2 Compared to the lastvertical test the cavity lost about 30 of its performancebecause of increased field emission and degraded againafter the first introduction of a Cs2Te cathode [18]

Nevertheless the remaining gradient is still higher thanthat of SRF gun I Over the past 5 years of operation thecavity performance has remained constant despite manyCs2Te and Mg cathodes that have been used in the gunSRF gun II is equipped with the same double-tuning

system as its predecessor Thus the half-cell can be tunedseparately from the three TESLA cells [9] Both tunershave been characterized at the cold state where they showan excellent frequency resolution of 03 Hz per step (half-cell) and 08 Hz per step (TESLA cells) as well asnegligible hysteresis At the same time the system providesa wide tuning range of almost300 kHz to compensate forvarious mechanical tolerances [19]Another three main characteristics of the cavity are

microphonics Lorentz force detuning and helium pressuresensitivity Despite all of the disturbances the total phasenoise measured using a Rhode amp Schwarz FSWP phasenoise analyzer from 10 Hz to 10 MHz is σφ frac14 001deg at13 GHz or σφ frac14 23 fs rms respectively and thus clearlyfulfills our requirements Lorentz force detuning leads to anoverall frequency shift of 650 Hz for the typical operatinggradient of Eacc frac14 8 MV=m The helium pressure sensitiv-ity of 150 Hz=mbar is almost exactly the same as for gun IThe photocathode itself is thermally and electrically

isolated from the cavity and cooled down with liquidnitrogen (Fig 3) That allows the application of normalconducting photocathodes in the superconducting environ-ment of the gun The cathode cooling and support unit wasdesigned for an easy exchange of the cathodes in the coldgun as well as for precise positioning Therefore threerotary feedthroughs at the back side of the module allow foraxial cathode movement (06 mm) with respect to thecavity for best rf focusing but also for radial adjustment ofthe cathode in the half-cell Additionally a dc bias can beapplied to the cathode in order to suppress multipacting

FIG 1 Cross section of the ELBE SRF gun II cryomodule holding a 35-cell gun cavity and a superconducting solenoid at about70 cm from the cathode

FIG 2 Comparison of the intrinsic quality factor Q0 as afunction of the accelerating gradient Eacc of SRF gun II Thediagram shows a significant improvement compared to SRF gunI even though not all of the earlier vertical performance could betransferred into routine operation

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Multipacting was one of the main problems for the SRFgun I As predicted by simulations the circular gapbetween the cathode and cavity was prone to multipactingBeside this geometrical reason the contamination of thephotocathode with cesium atoms arising from the Cs2Tedeposition process intensified the problem In cooperationwith the universities in Rostock and in Siegen [20]extensive studies were carried out to mitigate multipactingby additional surface layers or surface microstructuringFor the SRF gun II with Mg cathodes no special pre-cautions are needed as multipacting does not appear Inaddition even dark current typically on the order of 40 nAat 8 MV=m has no negative effect on the terahertz useroperation so it is not discussed in this paper However areference is given here in which the dark current issue isdiscussed in further detail [21]The cavity is completed by a coaxial fundamental mode

power coupler (FPC) and two TESLA-type higher-ordermode (HOM) dampers Thermal anchors made of copperbraids are used to improve the heat conduction betweenHOM couplers and the helium vessel Furthermore thesapphire rf feedthroughs of the HOM couplers can achievea higher thermal conductivity between the inner and outerconductor of the coaxial line Both reduce the thermal driftduring gradient changes and improve cw capability of thecavity substantially The FPC was developed at HZDR andcouples rf power via an open end (the antenna tip) into thefirst cell of the cavity The coupling factor (or external Q) isfixed in the range 6 times 106 lt Qext lt 3 times 107 The cold partof the coupler is composed by a coaxial waveguide whoseinner and outer conductors are connected to a conicalceramic cooled with liquid nitrogen The warm parts ofthe coupler realize the transition between the coaxial andthe rectangular waveguide used for low-loss rf transportA second warm window made of quartz glass is located inthe waveguide and acts as a vacuum barrier between thecoupler vacuum and atmosphere

B rf system and synchronization to drive laser

An amplitude and phase stable rf field for acceleration aswell as a low noise and low jitter locking of the photo-cathode laser to the rf reference are crucial for the stabilityand quality of the generated electron beam In particularfor superconducting niobium cavities with their frequencydetuning dominated by microphonics the former must berealized by active feedback loops For this purpose twoanalog proportional control loops for amplitude and phaseare combined into one controller (see Fig 4)Hence the resonator is driven at exactly 13 GHz

(generator-driven resonator) and both amplitude andphase are continuously adjusted by two actuators (volt-age-controlled phase shifter and attenuator) The transientsignal is taken by a cavity field probe and determined withrespect to amplitude (level detector) and phase (mixer)After low-pass filtering both are compared with the targetvalues provided by the programmable logic controller andsupervisory control and data acquisition system of theaccelerator The differential voltages determined in thisway control both actuators mentioned before The stabi-lized rf signal is then amplified by a solid state poweramplifier (SSPA) and brought to the cavity using a WR650waveguide To protect the amplifier a rf isolator is used todump the rf power that is reflected by the cavity into a rfload Additionally there is a possibility to adjust the cavitybandwidth by means of a three-stub waveguide tunerbetween 50 and 300 Hz The short-term amplitude stabilityof the amplitude control loop can reach 2 times 10minus4 rms andthe phase stability 001deg rms (measured between 10 Hzand 10 MHz)The required rf power is provided by a state-of-the-art

SSPA from Sigmaphi Electronics The 1300-MHz ampli-fier is composed by ten rf modules that are combined by aWR650 waveguide to reach up to 15 kW cw with abandwidth of 5 MHz Each of the rf modules delivers

FIG 3 Cathode holder and LN2 cooling system

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2 kW from four 500-W laterally-diffused metal-oxidesemiconductor (LDMOS) transistors organized around afour-way combiner and cooled by a copper plate Theguaranteed minimum small signal gain is 73 dB whereasthe expected efficiency at nominal rf power is 40ndash45The phase drift over an output power range of 2ndash15 kW isless than 8deg Gain linearity over the same dynamic range isless than 05 dB The rise and fall time is shorterthan 100 nsThe synchronization of the four linac cavities of ELBE

the SRF gun and the photocathode laser is realized using

the common master oscillator of 13 MHz and 13 GHz (seeFig 5) These frequencies are generated together with otherfrequencies required for operation by the RALAB accel-erator master clock from LAURIN AG The 13-GHzreference signal is divided by an active rf splitter ontothe low-level rf (LLRF) controllers of the harmonicbuncher the four ELBE linac cavities and the SRF gunEach of the latter two has a phase shifter along its signalpath to adjust the exact timing between the accelerator andgun A third phase shifter is used to change the timing ofthe laser with respect to gun rf In addition to the already-

FIG 4 Block diagram of the analog low-level rf controller

FIG 5 Block diagram of rf system and laser synchronization to the ELBE master clock

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mentioned high-frequency reference the laser gets threefurther multiples of 13 MHz provided by an opticaltransmission line and phase lock loops (PLLs) for 2652 and 182 MHz This guarantees a synchronous lockingof the laser oscillator to the 13-MHz master clock which isimportant for user experiments that use the same clock aswell The time jitter achieved with this setup is summarizedfor the 13-GHz reference the SRF gun rf and the laseroscillator in Table II

C Mg photocathodes

It is well known that the quality of the photocathodes iscritical for the stability and reliability of photoinjectoroperation In rf photoinjectors the survivability in rf fieldsis another major issue and therefore metal photocathodesare most suitable Combined with a UV drive laser a propermaterial is magnesium which has a low work function of36 eV and a quantum efficiency (QE) measured with258 nm UV light of up to 02 [22] which is the highestreported for metal photocathodesThe design of the photocathode for the SRF gun II is

presented in Fig 6 The cathode plug on top is a cylinderwith 10 mm diameter and 7ndash8 mm length which extends

into the superconducting cavity A bayonet fitting holds thecathode in the cooling system as described in Ref [9]The cone at the Cu body of the cathode together with theInconelreg spring realizes a good thermal contact and holdsthe cathode at LN2 temperatureThe cathode plug consists of pure 999 polycrystalline

Mg The plug is mirrorlike polished deoxidized cleanedand then stored in filtered dry N2 until installed in thecathode transport chamber After the chemical deoxidationprocess the QE of the Mg cathodes is only about 2 times 10minus5In order to reduce the surface work function the UV drivelaser is used for in-vacuum cleaning of the Mg surface with100 kHz repetition rate and 100 mW power The powerdensity of the laser beam for successful cleaning has to bearound 2 W=mm2 A specific optics setup has beendeveloped [2324] to perform the cleaning in the cathodetransport chamber of the cathode load-lock system For thatreason the transport chamber is equipped with fused silicawindows and a high-voltage feedthrough for an anodecollecting the photoemission electrons for QE measure-ment The cathodes have to be cleaned in the transportchamber in order to prevent any contamination of the guncavityThe laser cleaning leads to a QE improvement of 2

orders of magnitude reaching 02ndash04 The cleaningprocess is very well repeatable [25] so some of the Mgphotocathodes could be cleaned three times Then the oldMg plugs were replaced by new ones The procedure is toexchange the transport chamber at the gun about one timeper year during a shutdown time of ELBE Beforehandthe new chamber has been filled with three to fivephotocathodes with new Mg plugs polished and chemi-cally cleaned and inspected with respect to particlepollution or scratches in the clean room After chamberexchange the load-lock system at the gun is evacuatedand vacuum backed Finally two Mg cathodes are lasercleaned QEs are measured and the best cathode is loadedinto the gun The QE stays stable for a very long timenot only in the transport chamber with a vacuum ofle10minus10 mbar but also during SRF gun operation itselfThe cathode has always a shiny silver color but thesurface is changed as shown in Fig 7 The virgin part isthe polished mirrorlike surface while the cleaned partshows a periodic structure created by the scanning patternof the laser beamUp to now the study of Mg photocathodes has con-

centrated on issues affecting a stable and reliable gunoperation for terahertz users ie high QE and longlifetime Nevertheless a detailed study of the thermalemittance of Mg is still in progress Surface photoemissionplays an important role for Mg and thus the thermalemittance is expected to be a factor of 2 lower (05 μm=mm[22]) than predicted by the three-step model for metalsassuming volume photoemission [26] On the other handthe surface roughness of 100 nm with a period of 3ndash20 μm

TABLE II Rms timing jitter of rf reference SRF gun rf fieldand laser oscillator measured using Rhode amp Schwarz FSWPphase noise analyzer

Range13 GHzreference SRF gun rf

Laseroscillator

1ndash10 Hz 752 fs 1013 fs 1606 fs10 Hzndash10 MHz 273 fs 231 fs 857 fs1 Hzndash10 MHz 800 fs 1039 fs 1821 fs

FIG 6 Geometry of photocathode The insert shows a Mg plugphotograph

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produced by the laser increases the thermal emittanceApplying eg the two-dimensional model by Qian [27]on these values a growth by Δεth frac14 067ndash026 μm for14 MV=m cathode field and 1 mm rms laser spot size isexpected

D Drive laser system

The drive laser system for the SRF gun II has beendeveloped by Max Born Institute Berlin The laser has twochannels intended for different pulse repetition rates Onechannel is for 13 MHz repetition rate another for the range10ndash500 kHz Both channels produce Gaussian-like pulsesin the time domain of 3 and 5 ps FWHM length at 13 MHzand in the kilohertz region respectively Average outputpower is up to 05 W at 262 nmThe schematic layout of the drive laser system is shown

in Fig 8 The common part of the laser consists of a Ndglass mode-locked oscillator (52 MHz) followed by apulse picker that picks every fourth pulse thus reducing therepetition rate to 13 MHz Then the beam goes into either afiber amplifier or a regenerative amplifier in case ofoperating at 13 MHz or at 10ndash500 kHz respectively

The final amplification is provided by a multipass amplifierwhich is common for both channels At the end the systemsplits again into two frequency conversion modules Eachmodule consists of a lithium triborate (LBO) and a betabarium borate (BBO) crystal that consequently provideIR-to-green and green-to-UV conversion Because of a bigdifference in pulse energy at 13 MHz and at 10ndash500 kHzdifferent focusing is required to achieve the right peakpower density within the crystalsIn practice it turned out that the IR-to-UV conversion is

likely to be the main source of instabilities in the outputlaser beamWhen the IR part is rather stable there are still afew degrees of freedom to play with to optimize theconversion If not optimized the crystals can be too muchaffected by thermal load from the laser beam and therebydegrade quickly andor be too sensitive to small variationsin the IR input and in the environment A significantimprovement in both long-term power stability and spatialbeam profile (in the kilohertz module) has been achievedrather recently by modifying the beam configuration andplacement of the crystals Figure 9 illustrates the progressin the spatial beam profile

FIG 7 Photograph of Mg photocathode Mg214 after second laser cleaning (left) and surface structure changes after cleaning (opticalmicroscope image right)

FIG 8 Scheme of the two-channel laser developed by MBI Berlin

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After the laser an attenuator consisting of a quarter-waveplate and a polarizer is the first optical device on the UVbeam path The attenuator is used to set the required laserpower on the photocathode and to compensate for occa-sional power variations of the laser output Then a fastoptical mechanical shutter allows one to operate in so-calledldquomacropulse moderdquo (used when guiding and optimizing theelectron beam) where macropulses are 02ndash40 ms in lengthand their repetition rate is 1ndash10 Hz For the acceleratorsafety the beam must be shut down within 2 ms after adangerous level of electron beam loss has been detectedOwing to about 01 ms switching time the optical mechani-cal shutter allows one to satisfy this conditionVarious photodiodes and power meters and an autocor-

relator from APE GmbH Berlin (recently updated toperform autocorrelation directly in UV as well) are usedfor diagnostics and constant monitoring of the laser beamThe most recent autocorrelation measurement at UVconfirmed the pulse length of 23 ps rms (53 ps FWHM)The laser beam transport from the laser table to the

photocathode has a length of about 135 m A physicalaperture placed at the end of the optical table is imaged ontothe cathode with a magnification of 25 by two lensesAttention should be paid to using lenses and other

transmitting elements in a pulsed UV laser beam We haveencountered an effect when UV-grade fused silica experi-ences UV-induced compaction [28] becomes birefringentand thus progressively affects the beam profile There aredifferent grades of UV fused silica on the market and wehave observed that their resilience to UV laser radiationmay differ significantly At present we simply avoid usingstandard UV fused silica elements close to the laser outputThe first lens is placed 2 m after the beam-shaping aperturewhere the beam is attenuated and rather wide due to somegeometrical divergence which it has at the output of the UVconversion and thus also after the aperture Under theseconditions we have not observed any degradation of lensesthat perform imaging of the aperture on the cathode Thereis a row of apertures of different diameters placed on amotorized linear stage so that one can change between themremotely A motorized mirror allows one to finely positionthe laser spot on the cathode About 2 m before the cathodean uncoated wedge picks up about 5 of laser power forthe so-called virtual cathode The virtual cathode is a Ce-doped yttrium aluminum garnet (YAGCe) screen placed atthe same distance from the wedge as the real cathode so thatit allows one to see how the spot on the real cathode looksand how it moves over the cathode with 1∶1 ratio TheYAGCe screen is imaged on a CCD camera and fluorescesat green when being irradiated by the UV light

E Diagnostic beam line and connectionto ELBE linac

A diagnostics beam line is directly connected to the SRFgun [29] (see Fig 10) whereby its first part also serves forthe beam transport into the ELBE accelerator After thelaser input port a first screen station with a retractableFaraday cup is situated Following the beam direction threequadrupoles and a 45deg bending magnet for the ELBEconnection are installed In a straight direction the secondscreen station additionally contains a moving slit system fortransverse phase space and emittance measurements Aftera third screen station a 180deg spectrometer magnet is placed

FIG 9 Laser spot profile on the cathode before and afteroptimization of the UV conversion (the spot is 38 mm indiameter and is formed as an optical image of a physical apertureon the optical table)

FIG 10 The diagnostic beamline of the ELBE SRF gun II

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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higher bunch charge (200 pC) required for user operationat ELBEWithin a collaboration project the SRF gun II cavity was

fabricated treated and tested at the Thomas JeffersonNational Accelerator Facility (TJNAF) [17] The cavity wasvertically measured after final treatment at TJNAF in 2013and later in the gun cryomodule with a copper cathodeduring the commission process in 2014 Since then theperformance of cavity has been monitored regularly Someselected results are shown in Fig 2 Compared to the lastvertical test the cavity lost about 30 of its performancebecause of increased field emission and degraded againafter the first introduction of a Cs2Te cathode [18]

Nevertheless the remaining gradient is still higher thanthat of SRF gun I Over the past 5 years of operation thecavity performance has remained constant despite manyCs2Te and Mg cathodes that have been used in the gunSRF gun II is equipped with the same double-tuning

system as its predecessor Thus the half-cell can be tunedseparately from the three TESLA cells [9] Both tunershave been characterized at the cold state where they showan excellent frequency resolution of 03 Hz per step (half-cell) and 08 Hz per step (TESLA cells) as well asnegligible hysteresis At the same time the system providesa wide tuning range of almost300 kHz to compensate forvarious mechanical tolerances [19]Another three main characteristics of the cavity are

microphonics Lorentz force detuning and helium pressuresensitivity Despite all of the disturbances the total phasenoise measured using a Rhode amp Schwarz FSWP phasenoise analyzer from 10 Hz to 10 MHz is σφ frac14 001deg at13 GHz or σφ frac14 23 fs rms respectively and thus clearlyfulfills our requirements Lorentz force detuning leads to anoverall frequency shift of 650 Hz for the typical operatinggradient of Eacc frac14 8 MV=m The helium pressure sensitiv-ity of 150 Hz=mbar is almost exactly the same as for gun IThe photocathode itself is thermally and electrically

isolated from the cavity and cooled down with liquidnitrogen (Fig 3) That allows the application of normalconducting photocathodes in the superconducting environ-ment of the gun The cathode cooling and support unit wasdesigned for an easy exchange of the cathodes in the coldgun as well as for precise positioning Therefore threerotary feedthroughs at the back side of the module allow foraxial cathode movement (06 mm) with respect to thecavity for best rf focusing but also for radial adjustment ofthe cathode in the half-cell Additionally a dc bias can beapplied to the cathode in order to suppress multipacting

FIG 1 Cross section of the ELBE SRF gun II cryomodule holding a 35-cell gun cavity and a superconducting solenoid at about70 cm from the cathode

FIG 2 Comparison of the intrinsic quality factor Q0 as afunction of the accelerating gradient Eacc of SRF gun II Thediagram shows a significant improvement compared to SRF gunI even though not all of the earlier vertical performance could betransferred into routine operation

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Multipacting was one of the main problems for the SRFgun I As predicted by simulations the circular gapbetween the cathode and cavity was prone to multipactingBeside this geometrical reason the contamination of thephotocathode with cesium atoms arising from the Cs2Tedeposition process intensified the problem In cooperationwith the universities in Rostock and in Siegen [20]extensive studies were carried out to mitigate multipactingby additional surface layers or surface microstructuringFor the SRF gun II with Mg cathodes no special pre-cautions are needed as multipacting does not appear Inaddition even dark current typically on the order of 40 nAat 8 MV=m has no negative effect on the terahertz useroperation so it is not discussed in this paper However areference is given here in which the dark current issue isdiscussed in further detail [21]The cavity is completed by a coaxial fundamental mode

power coupler (FPC) and two TESLA-type higher-ordermode (HOM) dampers Thermal anchors made of copperbraids are used to improve the heat conduction betweenHOM couplers and the helium vessel Furthermore thesapphire rf feedthroughs of the HOM couplers can achievea higher thermal conductivity between the inner and outerconductor of the coaxial line Both reduce the thermal driftduring gradient changes and improve cw capability of thecavity substantially The FPC was developed at HZDR andcouples rf power via an open end (the antenna tip) into thefirst cell of the cavity The coupling factor (or external Q) isfixed in the range 6 times 106 lt Qext lt 3 times 107 The cold partof the coupler is composed by a coaxial waveguide whoseinner and outer conductors are connected to a conicalceramic cooled with liquid nitrogen The warm parts ofthe coupler realize the transition between the coaxial andthe rectangular waveguide used for low-loss rf transportA second warm window made of quartz glass is located inthe waveguide and acts as a vacuum barrier between thecoupler vacuum and atmosphere

B rf system and synchronization to drive laser

An amplitude and phase stable rf field for acceleration aswell as a low noise and low jitter locking of the photo-cathode laser to the rf reference are crucial for the stabilityand quality of the generated electron beam In particularfor superconducting niobium cavities with their frequencydetuning dominated by microphonics the former must berealized by active feedback loops For this purpose twoanalog proportional control loops for amplitude and phaseare combined into one controller (see Fig 4)Hence the resonator is driven at exactly 13 GHz

(generator-driven resonator) and both amplitude andphase are continuously adjusted by two actuators (volt-age-controlled phase shifter and attenuator) The transientsignal is taken by a cavity field probe and determined withrespect to amplitude (level detector) and phase (mixer)After low-pass filtering both are compared with the targetvalues provided by the programmable logic controller andsupervisory control and data acquisition system of theaccelerator The differential voltages determined in thisway control both actuators mentioned before The stabi-lized rf signal is then amplified by a solid state poweramplifier (SSPA) and brought to the cavity using a WR650waveguide To protect the amplifier a rf isolator is used todump the rf power that is reflected by the cavity into a rfload Additionally there is a possibility to adjust the cavitybandwidth by means of a three-stub waveguide tunerbetween 50 and 300 Hz The short-term amplitude stabilityof the amplitude control loop can reach 2 times 10minus4 rms andthe phase stability 001deg rms (measured between 10 Hzand 10 MHz)The required rf power is provided by a state-of-the-art

SSPA from Sigmaphi Electronics The 1300-MHz ampli-fier is composed by ten rf modules that are combined by aWR650 waveguide to reach up to 15 kW cw with abandwidth of 5 MHz Each of the rf modules delivers

FIG 3 Cathode holder and LN2 cooling system

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2 kW from four 500-W laterally-diffused metal-oxidesemiconductor (LDMOS) transistors organized around afour-way combiner and cooled by a copper plate Theguaranteed minimum small signal gain is 73 dB whereasthe expected efficiency at nominal rf power is 40ndash45The phase drift over an output power range of 2ndash15 kW isless than 8deg Gain linearity over the same dynamic range isless than 05 dB The rise and fall time is shorterthan 100 nsThe synchronization of the four linac cavities of ELBE

the SRF gun and the photocathode laser is realized using

the common master oscillator of 13 MHz and 13 GHz (seeFig 5) These frequencies are generated together with otherfrequencies required for operation by the RALAB accel-erator master clock from LAURIN AG The 13-GHzreference signal is divided by an active rf splitter ontothe low-level rf (LLRF) controllers of the harmonicbuncher the four ELBE linac cavities and the SRF gunEach of the latter two has a phase shifter along its signalpath to adjust the exact timing between the accelerator andgun A third phase shifter is used to change the timing ofthe laser with respect to gun rf In addition to the already-

FIG 4 Block diagram of the analog low-level rf controller

FIG 5 Block diagram of rf system and laser synchronization to the ELBE master clock

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mentioned high-frequency reference the laser gets threefurther multiples of 13 MHz provided by an opticaltransmission line and phase lock loops (PLLs) for 2652 and 182 MHz This guarantees a synchronous lockingof the laser oscillator to the 13-MHz master clock which isimportant for user experiments that use the same clock aswell The time jitter achieved with this setup is summarizedfor the 13-GHz reference the SRF gun rf and the laseroscillator in Table II

C Mg photocathodes

It is well known that the quality of the photocathodes iscritical for the stability and reliability of photoinjectoroperation In rf photoinjectors the survivability in rf fieldsis another major issue and therefore metal photocathodesare most suitable Combined with a UV drive laser a propermaterial is magnesium which has a low work function of36 eV and a quantum efficiency (QE) measured with258 nm UV light of up to 02 [22] which is the highestreported for metal photocathodesThe design of the photocathode for the SRF gun II is

presented in Fig 6 The cathode plug on top is a cylinderwith 10 mm diameter and 7ndash8 mm length which extends

into the superconducting cavity A bayonet fitting holds thecathode in the cooling system as described in Ref [9]The cone at the Cu body of the cathode together with theInconelreg spring realizes a good thermal contact and holdsthe cathode at LN2 temperatureThe cathode plug consists of pure 999 polycrystalline

Mg The plug is mirrorlike polished deoxidized cleanedand then stored in filtered dry N2 until installed in thecathode transport chamber After the chemical deoxidationprocess the QE of the Mg cathodes is only about 2 times 10minus5In order to reduce the surface work function the UV drivelaser is used for in-vacuum cleaning of the Mg surface with100 kHz repetition rate and 100 mW power The powerdensity of the laser beam for successful cleaning has to bearound 2 W=mm2 A specific optics setup has beendeveloped [2324] to perform the cleaning in the cathodetransport chamber of the cathode load-lock system For thatreason the transport chamber is equipped with fused silicawindows and a high-voltage feedthrough for an anodecollecting the photoemission electrons for QE measure-ment The cathodes have to be cleaned in the transportchamber in order to prevent any contamination of the guncavityThe laser cleaning leads to a QE improvement of 2

orders of magnitude reaching 02ndash04 The cleaningprocess is very well repeatable [25] so some of the Mgphotocathodes could be cleaned three times Then the oldMg plugs were replaced by new ones The procedure is toexchange the transport chamber at the gun about one timeper year during a shutdown time of ELBE Beforehandthe new chamber has been filled with three to fivephotocathodes with new Mg plugs polished and chemi-cally cleaned and inspected with respect to particlepollution or scratches in the clean room After chamberexchange the load-lock system at the gun is evacuatedand vacuum backed Finally two Mg cathodes are lasercleaned QEs are measured and the best cathode is loadedinto the gun The QE stays stable for a very long timenot only in the transport chamber with a vacuum ofle10minus10 mbar but also during SRF gun operation itselfThe cathode has always a shiny silver color but thesurface is changed as shown in Fig 7 The virgin part isthe polished mirrorlike surface while the cleaned partshows a periodic structure created by the scanning patternof the laser beamUp to now the study of Mg photocathodes has con-

centrated on issues affecting a stable and reliable gunoperation for terahertz users ie high QE and longlifetime Nevertheless a detailed study of the thermalemittance of Mg is still in progress Surface photoemissionplays an important role for Mg and thus the thermalemittance is expected to be a factor of 2 lower (05 μm=mm[22]) than predicted by the three-step model for metalsassuming volume photoemission [26] On the other handthe surface roughness of 100 nm with a period of 3ndash20 μm

TABLE II Rms timing jitter of rf reference SRF gun rf fieldand laser oscillator measured using Rhode amp Schwarz FSWPphase noise analyzer

Range13 GHzreference SRF gun rf

Laseroscillator

1ndash10 Hz 752 fs 1013 fs 1606 fs10 Hzndash10 MHz 273 fs 231 fs 857 fs1 Hzndash10 MHz 800 fs 1039 fs 1821 fs

FIG 6 Geometry of photocathode The insert shows a Mg plugphotograph

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produced by the laser increases the thermal emittanceApplying eg the two-dimensional model by Qian [27]on these values a growth by Δεth frac14 067ndash026 μm for14 MV=m cathode field and 1 mm rms laser spot size isexpected

D Drive laser system

The drive laser system for the SRF gun II has beendeveloped by Max Born Institute Berlin The laser has twochannels intended for different pulse repetition rates Onechannel is for 13 MHz repetition rate another for the range10ndash500 kHz Both channels produce Gaussian-like pulsesin the time domain of 3 and 5 ps FWHM length at 13 MHzand in the kilohertz region respectively Average outputpower is up to 05 W at 262 nmThe schematic layout of the drive laser system is shown

in Fig 8 The common part of the laser consists of a Ndglass mode-locked oscillator (52 MHz) followed by apulse picker that picks every fourth pulse thus reducing therepetition rate to 13 MHz Then the beam goes into either afiber amplifier or a regenerative amplifier in case ofoperating at 13 MHz or at 10ndash500 kHz respectively

The final amplification is provided by a multipass amplifierwhich is common for both channels At the end the systemsplits again into two frequency conversion modules Eachmodule consists of a lithium triborate (LBO) and a betabarium borate (BBO) crystal that consequently provideIR-to-green and green-to-UV conversion Because of a bigdifference in pulse energy at 13 MHz and at 10ndash500 kHzdifferent focusing is required to achieve the right peakpower density within the crystalsIn practice it turned out that the IR-to-UV conversion is

likely to be the main source of instabilities in the outputlaser beamWhen the IR part is rather stable there are still afew degrees of freedom to play with to optimize theconversion If not optimized the crystals can be too muchaffected by thermal load from the laser beam and therebydegrade quickly andor be too sensitive to small variationsin the IR input and in the environment A significantimprovement in both long-term power stability and spatialbeam profile (in the kilohertz module) has been achievedrather recently by modifying the beam configuration andplacement of the crystals Figure 9 illustrates the progressin the spatial beam profile

FIG 7 Photograph of Mg photocathode Mg214 after second laser cleaning (left) and surface structure changes after cleaning (opticalmicroscope image right)

FIG 8 Scheme of the two-channel laser developed by MBI Berlin

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After the laser an attenuator consisting of a quarter-waveplate and a polarizer is the first optical device on the UVbeam path The attenuator is used to set the required laserpower on the photocathode and to compensate for occa-sional power variations of the laser output Then a fastoptical mechanical shutter allows one to operate in so-calledldquomacropulse moderdquo (used when guiding and optimizing theelectron beam) where macropulses are 02ndash40 ms in lengthand their repetition rate is 1ndash10 Hz For the acceleratorsafety the beam must be shut down within 2 ms after adangerous level of electron beam loss has been detectedOwing to about 01 ms switching time the optical mechani-cal shutter allows one to satisfy this conditionVarious photodiodes and power meters and an autocor-

relator from APE GmbH Berlin (recently updated toperform autocorrelation directly in UV as well) are usedfor diagnostics and constant monitoring of the laser beamThe most recent autocorrelation measurement at UVconfirmed the pulse length of 23 ps rms (53 ps FWHM)The laser beam transport from the laser table to the

photocathode has a length of about 135 m A physicalaperture placed at the end of the optical table is imaged ontothe cathode with a magnification of 25 by two lensesAttention should be paid to using lenses and other

transmitting elements in a pulsed UV laser beam We haveencountered an effect when UV-grade fused silica experi-ences UV-induced compaction [28] becomes birefringentand thus progressively affects the beam profile There aredifferent grades of UV fused silica on the market and wehave observed that their resilience to UV laser radiationmay differ significantly At present we simply avoid usingstandard UV fused silica elements close to the laser outputThe first lens is placed 2 m after the beam-shaping aperturewhere the beam is attenuated and rather wide due to somegeometrical divergence which it has at the output of the UVconversion and thus also after the aperture Under theseconditions we have not observed any degradation of lensesthat perform imaging of the aperture on the cathode Thereis a row of apertures of different diameters placed on amotorized linear stage so that one can change between themremotely A motorized mirror allows one to finely positionthe laser spot on the cathode About 2 m before the cathodean uncoated wedge picks up about 5 of laser power forthe so-called virtual cathode The virtual cathode is a Ce-doped yttrium aluminum garnet (YAGCe) screen placed atthe same distance from the wedge as the real cathode so thatit allows one to see how the spot on the real cathode looksand how it moves over the cathode with 1∶1 ratio TheYAGCe screen is imaged on a CCD camera and fluorescesat green when being irradiated by the UV light

E Diagnostic beam line and connectionto ELBE linac

A diagnostics beam line is directly connected to the SRFgun [29] (see Fig 10) whereby its first part also serves forthe beam transport into the ELBE accelerator After thelaser input port a first screen station with a retractableFaraday cup is situated Following the beam direction threequadrupoles and a 45deg bending magnet for the ELBEconnection are installed In a straight direction the secondscreen station additionally contains a moving slit system fortransverse phase space and emittance measurements Aftera third screen station a 180deg spectrometer magnet is placed

FIG 9 Laser spot profile on the cathode before and afteroptimization of the UV conversion (the spot is 38 mm indiameter and is formed as an optical image of a physical apertureon the optical table)

FIG 10 The diagnostic beamline of the ELBE SRF gun II

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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Multipacting was one of the main problems for the SRFgun I As predicted by simulations the circular gapbetween the cathode and cavity was prone to multipactingBeside this geometrical reason the contamination of thephotocathode with cesium atoms arising from the Cs2Tedeposition process intensified the problem In cooperationwith the universities in Rostock and in Siegen [20]extensive studies were carried out to mitigate multipactingby additional surface layers or surface microstructuringFor the SRF gun II with Mg cathodes no special pre-cautions are needed as multipacting does not appear Inaddition even dark current typically on the order of 40 nAat 8 MV=m has no negative effect on the terahertz useroperation so it is not discussed in this paper However areference is given here in which the dark current issue isdiscussed in further detail [21]The cavity is completed by a coaxial fundamental mode

power coupler (FPC) and two TESLA-type higher-ordermode (HOM) dampers Thermal anchors made of copperbraids are used to improve the heat conduction betweenHOM couplers and the helium vessel Furthermore thesapphire rf feedthroughs of the HOM couplers can achievea higher thermal conductivity between the inner and outerconductor of the coaxial line Both reduce the thermal driftduring gradient changes and improve cw capability of thecavity substantially The FPC was developed at HZDR andcouples rf power via an open end (the antenna tip) into thefirst cell of the cavity The coupling factor (or external Q) isfixed in the range 6 times 106 lt Qext lt 3 times 107 The cold partof the coupler is composed by a coaxial waveguide whoseinner and outer conductors are connected to a conicalceramic cooled with liquid nitrogen The warm parts ofthe coupler realize the transition between the coaxial andthe rectangular waveguide used for low-loss rf transportA second warm window made of quartz glass is located inthe waveguide and acts as a vacuum barrier between thecoupler vacuum and atmosphere

B rf system and synchronization to drive laser

An amplitude and phase stable rf field for acceleration aswell as a low noise and low jitter locking of the photo-cathode laser to the rf reference are crucial for the stabilityand quality of the generated electron beam In particularfor superconducting niobium cavities with their frequencydetuning dominated by microphonics the former must berealized by active feedback loops For this purpose twoanalog proportional control loops for amplitude and phaseare combined into one controller (see Fig 4)Hence the resonator is driven at exactly 13 GHz

(generator-driven resonator) and both amplitude andphase are continuously adjusted by two actuators (volt-age-controlled phase shifter and attenuator) The transientsignal is taken by a cavity field probe and determined withrespect to amplitude (level detector) and phase (mixer)After low-pass filtering both are compared with the targetvalues provided by the programmable logic controller andsupervisory control and data acquisition system of theaccelerator The differential voltages determined in thisway control both actuators mentioned before The stabi-lized rf signal is then amplified by a solid state poweramplifier (SSPA) and brought to the cavity using a WR650waveguide To protect the amplifier a rf isolator is used todump the rf power that is reflected by the cavity into a rfload Additionally there is a possibility to adjust the cavitybandwidth by means of a three-stub waveguide tunerbetween 50 and 300 Hz The short-term amplitude stabilityof the amplitude control loop can reach 2 times 10minus4 rms andthe phase stability 001deg rms (measured between 10 Hzand 10 MHz)The required rf power is provided by a state-of-the-art

SSPA from Sigmaphi Electronics The 1300-MHz ampli-fier is composed by ten rf modules that are combined by aWR650 waveguide to reach up to 15 kW cw with abandwidth of 5 MHz Each of the rf modules delivers

FIG 3 Cathode holder and LN2 cooling system

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2 kW from four 500-W laterally-diffused metal-oxidesemiconductor (LDMOS) transistors organized around afour-way combiner and cooled by a copper plate Theguaranteed minimum small signal gain is 73 dB whereasthe expected efficiency at nominal rf power is 40ndash45The phase drift over an output power range of 2ndash15 kW isless than 8deg Gain linearity over the same dynamic range isless than 05 dB The rise and fall time is shorterthan 100 nsThe synchronization of the four linac cavities of ELBE

the SRF gun and the photocathode laser is realized using

the common master oscillator of 13 MHz and 13 GHz (seeFig 5) These frequencies are generated together with otherfrequencies required for operation by the RALAB accel-erator master clock from LAURIN AG The 13-GHzreference signal is divided by an active rf splitter ontothe low-level rf (LLRF) controllers of the harmonicbuncher the four ELBE linac cavities and the SRF gunEach of the latter two has a phase shifter along its signalpath to adjust the exact timing between the accelerator andgun A third phase shifter is used to change the timing ofthe laser with respect to gun rf In addition to the already-

FIG 4 Block diagram of the analog low-level rf controller

FIG 5 Block diagram of rf system and laser synchronization to the ELBE master clock

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mentioned high-frequency reference the laser gets threefurther multiples of 13 MHz provided by an opticaltransmission line and phase lock loops (PLLs) for 2652 and 182 MHz This guarantees a synchronous lockingof the laser oscillator to the 13-MHz master clock which isimportant for user experiments that use the same clock aswell The time jitter achieved with this setup is summarizedfor the 13-GHz reference the SRF gun rf and the laseroscillator in Table II

C Mg photocathodes

It is well known that the quality of the photocathodes iscritical for the stability and reliability of photoinjectoroperation In rf photoinjectors the survivability in rf fieldsis another major issue and therefore metal photocathodesare most suitable Combined with a UV drive laser a propermaterial is magnesium which has a low work function of36 eV and a quantum efficiency (QE) measured with258 nm UV light of up to 02 [22] which is the highestreported for metal photocathodesThe design of the photocathode for the SRF gun II is

presented in Fig 6 The cathode plug on top is a cylinderwith 10 mm diameter and 7ndash8 mm length which extends

into the superconducting cavity A bayonet fitting holds thecathode in the cooling system as described in Ref [9]The cone at the Cu body of the cathode together with theInconelreg spring realizes a good thermal contact and holdsthe cathode at LN2 temperatureThe cathode plug consists of pure 999 polycrystalline

Mg The plug is mirrorlike polished deoxidized cleanedand then stored in filtered dry N2 until installed in thecathode transport chamber After the chemical deoxidationprocess the QE of the Mg cathodes is only about 2 times 10minus5In order to reduce the surface work function the UV drivelaser is used for in-vacuum cleaning of the Mg surface with100 kHz repetition rate and 100 mW power The powerdensity of the laser beam for successful cleaning has to bearound 2 W=mm2 A specific optics setup has beendeveloped [2324] to perform the cleaning in the cathodetransport chamber of the cathode load-lock system For thatreason the transport chamber is equipped with fused silicawindows and a high-voltage feedthrough for an anodecollecting the photoemission electrons for QE measure-ment The cathodes have to be cleaned in the transportchamber in order to prevent any contamination of the guncavityThe laser cleaning leads to a QE improvement of 2

orders of magnitude reaching 02ndash04 The cleaningprocess is very well repeatable [25] so some of the Mgphotocathodes could be cleaned three times Then the oldMg plugs were replaced by new ones The procedure is toexchange the transport chamber at the gun about one timeper year during a shutdown time of ELBE Beforehandthe new chamber has been filled with three to fivephotocathodes with new Mg plugs polished and chemi-cally cleaned and inspected with respect to particlepollution or scratches in the clean room After chamberexchange the load-lock system at the gun is evacuatedand vacuum backed Finally two Mg cathodes are lasercleaned QEs are measured and the best cathode is loadedinto the gun The QE stays stable for a very long timenot only in the transport chamber with a vacuum ofle10minus10 mbar but also during SRF gun operation itselfThe cathode has always a shiny silver color but thesurface is changed as shown in Fig 7 The virgin part isthe polished mirrorlike surface while the cleaned partshows a periodic structure created by the scanning patternof the laser beamUp to now the study of Mg photocathodes has con-

centrated on issues affecting a stable and reliable gunoperation for terahertz users ie high QE and longlifetime Nevertheless a detailed study of the thermalemittance of Mg is still in progress Surface photoemissionplays an important role for Mg and thus the thermalemittance is expected to be a factor of 2 lower (05 μm=mm[22]) than predicted by the three-step model for metalsassuming volume photoemission [26] On the other handthe surface roughness of 100 nm with a period of 3ndash20 μm

TABLE II Rms timing jitter of rf reference SRF gun rf fieldand laser oscillator measured using Rhode amp Schwarz FSWPphase noise analyzer

Range13 GHzreference SRF gun rf

Laseroscillator

1ndash10 Hz 752 fs 1013 fs 1606 fs10 Hzndash10 MHz 273 fs 231 fs 857 fs1 Hzndash10 MHz 800 fs 1039 fs 1821 fs

FIG 6 Geometry of photocathode The insert shows a Mg plugphotograph

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produced by the laser increases the thermal emittanceApplying eg the two-dimensional model by Qian [27]on these values a growth by Δεth frac14 067ndash026 μm for14 MV=m cathode field and 1 mm rms laser spot size isexpected

D Drive laser system

The drive laser system for the SRF gun II has beendeveloped by Max Born Institute Berlin The laser has twochannels intended for different pulse repetition rates Onechannel is for 13 MHz repetition rate another for the range10ndash500 kHz Both channels produce Gaussian-like pulsesin the time domain of 3 and 5 ps FWHM length at 13 MHzand in the kilohertz region respectively Average outputpower is up to 05 W at 262 nmThe schematic layout of the drive laser system is shown

in Fig 8 The common part of the laser consists of a Ndglass mode-locked oscillator (52 MHz) followed by apulse picker that picks every fourth pulse thus reducing therepetition rate to 13 MHz Then the beam goes into either afiber amplifier or a regenerative amplifier in case ofoperating at 13 MHz or at 10ndash500 kHz respectively

The final amplification is provided by a multipass amplifierwhich is common for both channels At the end the systemsplits again into two frequency conversion modules Eachmodule consists of a lithium triborate (LBO) and a betabarium borate (BBO) crystal that consequently provideIR-to-green and green-to-UV conversion Because of a bigdifference in pulse energy at 13 MHz and at 10ndash500 kHzdifferent focusing is required to achieve the right peakpower density within the crystalsIn practice it turned out that the IR-to-UV conversion is

likely to be the main source of instabilities in the outputlaser beamWhen the IR part is rather stable there are still afew degrees of freedom to play with to optimize theconversion If not optimized the crystals can be too muchaffected by thermal load from the laser beam and therebydegrade quickly andor be too sensitive to small variationsin the IR input and in the environment A significantimprovement in both long-term power stability and spatialbeam profile (in the kilohertz module) has been achievedrather recently by modifying the beam configuration andplacement of the crystals Figure 9 illustrates the progressin the spatial beam profile

FIG 7 Photograph of Mg photocathode Mg214 after second laser cleaning (left) and surface structure changes after cleaning (opticalmicroscope image right)

FIG 8 Scheme of the two-channel laser developed by MBI Berlin

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After the laser an attenuator consisting of a quarter-waveplate and a polarizer is the first optical device on the UVbeam path The attenuator is used to set the required laserpower on the photocathode and to compensate for occa-sional power variations of the laser output Then a fastoptical mechanical shutter allows one to operate in so-calledldquomacropulse moderdquo (used when guiding and optimizing theelectron beam) where macropulses are 02ndash40 ms in lengthand their repetition rate is 1ndash10 Hz For the acceleratorsafety the beam must be shut down within 2 ms after adangerous level of electron beam loss has been detectedOwing to about 01 ms switching time the optical mechani-cal shutter allows one to satisfy this conditionVarious photodiodes and power meters and an autocor-

relator from APE GmbH Berlin (recently updated toperform autocorrelation directly in UV as well) are usedfor diagnostics and constant monitoring of the laser beamThe most recent autocorrelation measurement at UVconfirmed the pulse length of 23 ps rms (53 ps FWHM)The laser beam transport from the laser table to the

photocathode has a length of about 135 m A physicalaperture placed at the end of the optical table is imaged ontothe cathode with a magnification of 25 by two lensesAttention should be paid to using lenses and other

transmitting elements in a pulsed UV laser beam We haveencountered an effect when UV-grade fused silica experi-ences UV-induced compaction [28] becomes birefringentand thus progressively affects the beam profile There aredifferent grades of UV fused silica on the market and wehave observed that their resilience to UV laser radiationmay differ significantly At present we simply avoid usingstandard UV fused silica elements close to the laser outputThe first lens is placed 2 m after the beam-shaping aperturewhere the beam is attenuated and rather wide due to somegeometrical divergence which it has at the output of the UVconversion and thus also after the aperture Under theseconditions we have not observed any degradation of lensesthat perform imaging of the aperture on the cathode Thereis a row of apertures of different diameters placed on amotorized linear stage so that one can change between themremotely A motorized mirror allows one to finely positionthe laser spot on the cathode About 2 m before the cathodean uncoated wedge picks up about 5 of laser power forthe so-called virtual cathode The virtual cathode is a Ce-doped yttrium aluminum garnet (YAGCe) screen placed atthe same distance from the wedge as the real cathode so thatit allows one to see how the spot on the real cathode looksand how it moves over the cathode with 1∶1 ratio TheYAGCe screen is imaged on a CCD camera and fluorescesat green when being irradiated by the UV light

E Diagnostic beam line and connectionto ELBE linac

A diagnostics beam line is directly connected to the SRFgun [29] (see Fig 10) whereby its first part also serves forthe beam transport into the ELBE accelerator After thelaser input port a first screen station with a retractableFaraday cup is situated Following the beam direction threequadrupoles and a 45deg bending magnet for the ELBEconnection are installed In a straight direction the secondscreen station additionally contains a moving slit system fortransverse phase space and emittance measurements Aftera third screen station a 180deg spectrometer magnet is placed

FIG 9 Laser spot profile on the cathode before and afteroptimization of the UV conversion (the spot is 38 mm indiameter and is formed as an optical image of a physical apertureon the optical table)

FIG 10 The diagnostic beamline of the ELBE SRF gun II

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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2 kW from four 500-W laterally-diffused metal-oxidesemiconductor (LDMOS) transistors organized around afour-way combiner and cooled by a copper plate Theguaranteed minimum small signal gain is 73 dB whereasthe expected efficiency at nominal rf power is 40ndash45The phase drift over an output power range of 2ndash15 kW isless than 8deg Gain linearity over the same dynamic range isless than 05 dB The rise and fall time is shorterthan 100 nsThe synchronization of the four linac cavities of ELBE

the SRF gun and the photocathode laser is realized using

the common master oscillator of 13 MHz and 13 GHz (seeFig 5) These frequencies are generated together with otherfrequencies required for operation by the RALAB accel-erator master clock from LAURIN AG The 13-GHzreference signal is divided by an active rf splitter ontothe low-level rf (LLRF) controllers of the harmonicbuncher the four ELBE linac cavities and the SRF gunEach of the latter two has a phase shifter along its signalpath to adjust the exact timing between the accelerator andgun A third phase shifter is used to change the timing ofthe laser with respect to gun rf In addition to the already-

FIG 4 Block diagram of the analog low-level rf controller

FIG 5 Block diagram of rf system and laser synchronization to the ELBE master clock

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mentioned high-frequency reference the laser gets threefurther multiples of 13 MHz provided by an opticaltransmission line and phase lock loops (PLLs) for 2652 and 182 MHz This guarantees a synchronous lockingof the laser oscillator to the 13-MHz master clock which isimportant for user experiments that use the same clock aswell The time jitter achieved with this setup is summarizedfor the 13-GHz reference the SRF gun rf and the laseroscillator in Table II

C Mg photocathodes

It is well known that the quality of the photocathodes iscritical for the stability and reliability of photoinjectoroperation In rf photoinjectors the survivability in rf fieldsis another major issue and therefore metal photocathodesare most suitable Combined with a UV drive laser a propermaterial is magnesium which has a low work function of36 eV and a quantum efficiency (QE) measured with258 nm UV light of up to 02 [22] which is the highestreported for metal photocathodesThe design of the photocathode for the SRF gun II is

presented in Fig 6 The cathode plug on top is a cylinderwith 10 mm diameter and 7ndash8 mm length which extends

into the superconducting cavity A bayonet fitting holds thecathode in the cooling system as described in Ref [9]The cone at the Cu body of the cathode together with theInconelreg spring realizes a good thermal contact and holdsthe cathode at LN2 temperatureThe cathode plug consists of pure 999 polycrystalline

Mg The plug is mirrorlike polished deoxidized cleanedand then stored in filtered dry N2 until installed in thecathode transport chamber After the chemical deoxidationprocess the QE of the Mg cathodes is only about 2 times 10minus5In order to reduce the surface work function the UV drivelaser is used for in-vacuum cleaning of the Mg surface with100 kHz repetition rate and 100 mW power The powerdensity of the laser beam for successful cleaning has to bearound 2 W=mm2 A specific optics setup has beendeveloped [2324] to perform the cleaning in the cathodetransport chamber of the cathode load-lock system For thatreason the transport chamber is equipped with fused silicawindows and a high-voltage feedthrough for an anodecollecting the photoemission electrons for QE measure-ment The cathodes have to be cleaned in the transportchamber in order to prevent any contamination of the guncavityThe laser cleaning leads to a QE improvement of 2

orders of magnitude reaching 02ndash04 The cleaningprocess is very well repeatable [25] so some of the Mgphotocathodes could be cleaned three times Then the oldMg plugs were replaced by new ones The procedure is toexchange the transport chamber at the gun about one timeper year during a shutdown time of ELBE Beforehandthe new chamber has been filled with three to fivephotocathodes with new Mg plugs polished and chemi-cally cleaned and inspected with respect to particlepollution or scratches in the clean room After chamberexchange the load-lock system at the gun is evacuatedand vacuum backed Finally two Mg cathodes are lasercleaned QEs are measured and the best cathode is loadedinto the gun The QE stays stable for a very long timenot only in the transport chamber with a vacuum ofle10minus10 mbar but also during SRF gun operation itselfThe cathode has always a shiny silver color but thesurface is changed as shown in Fig 7 The virgin part isthe polished mirrorlike surface while the cleaned partshows a periodic structure created by the scanning patternof the laser beamUp to now the study of Mg photocathodes has con-

centrated on issues affecting a stable and reliable gunoperation for terahertz users ie high QE and longlifetime Nevertheless a detailed study of the thermalemittance of Mg is still in progress Surface photoemissionplays an important role for Mg and thus the thermalemittance is expected to be a factor of 2 lower (05 μm=mm[22]) than predicted by the three-step model for metalsassuming volume photoemission [26] On the other handthe surface roughness of 100 nm with a period of 3ndash20 μm

TABLE II Rms timing jitter of rf reference SRF gun rf fieldand laser oscillator measured using Rhode amp Schwarz FSWPphase noise analyzer

Range13 GHzreference SRF gun rf

Laseroscillator

1ndash10 Hz 752 fs 1013 fs 1606 fs10 Hzndash10 MHz 273 fs 231 fs 857 fs1 Hzndash10 MHz 800 fs 1039 fs 1821 fs

FIG 6 Geometry of photocathode The insert shows a Mg plugphotograph

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produced by the laser increases the thermal emittanceApplying eg the two-dimensional model by Qian [27]on these values a growth by Δεth frac14 067ndash026 μm for14 MV=m cathode field and 1 mm rms laser spot size isexpected

D Drive laser system

The drive laser system for the SRF gun II has beendeveloped by Max Born Institute Berlin The laser has twochannels intended for different pulse repetition rates Onechannel is for 13 MHz repetition rate another for the range10ndash500 kHz Both channels produce Gaussian-like pulsesin the time domain of 3 and 5 ps FWHM length at 13 MHzand in the kilohertz region respectively Average outputpower is up to 05 W at 262 nmThe schematic layout of the drive laser system is shown

in Fig 8 The common part of the laser consists of a Ndglass mode-locked oscillator (52 MHz) followed by apulse picker that picks every fourth pulse thus reducing therepetition rate to 13 MHz Then the beam goes into either afiber amplifier or a regenerative amplifier in case ofoperating at 13 MHz or at 10ndash500 kHz respectively

The final amplification is provided by a multipass amplifierwhich is common for both channels At the end the systemsplits again into two frequency conversion modules Eachmodule consists of a lithium triborate (LBO) and a betabarium borate (BBO) crystal that consequently provideIR-to-green and green-to-UV conversion Because of a bigdifference in pulse energy at 13 MHz and at 10ndash500 kHzdifferent focusing is required to achieve the right peakpower density within the crystalsIn practice it turned out that the IR-to-UV conversion is

likely to be the main source of instabilities in the outputlaser beamWhen the IR part is rather stable there are still afew degrees of freedom to play with to optimize theconversion If not optimized the crystals can be too muchaffected by thermal load from the laser beam and therebydegrade quickly andor be too sensitive to small variationsin the IR input and in the environment A significantimprovement in both long-term power stability and spatialbeam profile (in the kilohertz module) has been achievedrather recently by modifying the beam configuration andplacement of the crystals Figure 9 illustrates the progressin the spatial beam profile

FIG 7 Photograph of Mg photocathode Mg214 after second laser cleaning (left) and surface structure changes after cleaning (opticalmicroscope image right)

FIG 8 Scheme of the two-channel laser developed by MBI Berlin

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After the laser an attenuator consisting of a quarter-waveplate and a polarizer is the first optical device on the UVbeam path The attenuator is used to set the required laserpower on the photocathode and to compensate for occa-sional power variations of the laser output Then a fastoptical mechanical shutter allows one to operate in so-calledldquomacropulse moderdquo (used when guiding and optimizing theelectron beam) where macropulses are 02ndash40 ms in lengthand their repetition rate is 1ndash10 Hz For the acceleratorsafety the beam must be shut down within 2 ms after adangerous level of electron beam loss has been detectedOwing to about 01 ms switching time the optical mechani-cal shutter allows one to satisfy this conditionVarious photodiodes and power meters and an autocor-

relator from APE GmbH Berlin (recently updated toperform autocorrelation directly in UV as well) are usedfor diagnostics and constant monitoring of the laser beamThe most recent autocorrelation measurement at UVconfirmed the pulse length of 23 ps rms (53 ps FWHM)The laser beam transport from the laser table to the

photocathode has a length of about 135 m A physicalaperture placed at the end of the optical table is imaged ontothe cathode with a magnification of 25 by two lensesAttention should be paid to using lenses and other

transmitting elements in a pulsed UV laser beam We haveencountered an effect when UV-grade fused silica experi-ences UV-induced compaction [28] becomes birefringentand thus progressively affects the beam profile There aredifferent grades of UV fused silica on the market and wehave observed that their resilience to UV laser radiationmay differ significantly At present we simply avoid usingstandard UV fused silica elements close to the laser outputThe first lens is placed 2 m after the beam-shaping aperturewhere the beam is attenuated and rather wide due to somegeometrical divergence which it has at the output of the UVconversion and thus also after the aperture Under theseconditions we have not observed any degradation of lensesthat perform imaging of the aperture on the cathode Thereis a row of apertures of different diameters placed on amotorized linear stage so that one can change between themremotely A motorized mirror allows one to finely positionthe laser spot on the cathode About 2 m before the cathodean uncoated wedge picks up about 5 of laser power forthe so-called virtual cathode The virtual cathode is a Ce-doped yttrium aluminum garnet (YAGCe) screen placed atthe same distance from the wedge as the real cathode so thatit allows one to see how the spot on the real cathode looksand how it moves over the cathode with 1∶1 ratio TheYAGCe screen is imaged on a CCD camera and fluorescesat green when being irradiated by the UV light

E Diagnostic beam line and connectionto ELBE linac

A diagnostics beam line is directly connected to the SRFgun [29] (see Fig 10) whereby its first part also serves forthe beam transport into the ELBE accelerator After thelaser input port a first screen station with a retractableFaraday cup is situated Following the beam direction threequadrupoles and a 45deg bending magnet for the ELBEconnection are installed In a straight direction the secondscreen station additionally contains a moving slit system fortransverse phase space and emittance measurements Aftera third screen station a 180deg spectrometer magnet is placed

FIG 9 Laser spot profile on the cathode before and afteroptimization of the UV conversion (the spot is 38 mm indiameter and is formed as an optical image of a physical apertureon the optical table)

FIG 10 The diagnostic beamline of the ELBE SRF gun II

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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mentioned high-frequency reference the laser gets threefurther multiples of 13 MHz provided by an opticaltransmission line and phase lock loops (PLLs) for 2652 and 182 MHz This guarantees a synchronous lockingof the laser oscillator to the 13-MHz master clock which isimportant for user experiments that use the same clock aswell The time jitter achieved with this setup is summarizedfor the 13-GHz reference the SRF gun rf and the laseroscillator in Table II

C Mg photocathodes

It is well known that the quality of the photocathodes iscritical for the stability and reliability of photoinjectoroperation In rf photoinjectors the survivability in rf fieldsis another major issue and therefore metal photocathodesare most suitable Combined with a UV drive laser a propermaterial is magnesium which has a low work function of36 eV and a quantum efficiency (QE) measured with258 nm UV light of up to 02 [22] which is the highestreported for metal photocathodesThe design of the photocathode for the SRF gun II is

presented in Fig 6 The cathode plug on top is a cylinderwith 10 mm diameter and 7ndash8 mm length which extends

into the superconducting cavity A bayonet fitting holds thecathode in the cooling system as described in Ref [9]The cone at the Cu body of the cathode together with theInconelreg spring realizes a good thermal contact and holdsthe cathode at LN2 temperatureThe cathode plug consists of pure 999 polycrystalline

Mg The plug is mirrorlike polished deoxidized cleanedand then stored in filtered dry N2 until installed in thecathode transport chamber After the chemical deoxidationprocess the QE of the Mg cathodes is only about 2 times 10minus5In order to reduce the surface work function the UV drivelaser is used for in-vacuum cleaning of the Mg surface with100 kHz repetition rate and 100 mW power The powerdensity of the laser beam for successful cleaning has to bearound 2 W=mm2 A specific optics setup has beendeveloped [2324] to perform the cleaning in the cathodetransport chamber of the cathode load-lock system For thatreason the transport chamber is equipped with fused silicawindows and a high-voltage feedthrough for an anodecollecting the photoemission electrons for QE measure-ment The cathodes have to be cleaned in the transportchamber in order to prevent any contamination of the guncavityThe laser cleaning leads to a QE improvement of 2

orders of magnitude reaching 02ndash04 The cleaningprocess is very well repeatable [25] so some of the Mgphotocathodes could be cleaned three times Then the oldMg plugs were replaced by new ones The procedure is toexchange the transport chamber at the gun about one timeper year during a shutdown time of ELBE Beforehandthe new chamber has been filled with three to fivephotocathodes with new Mg plugs polished and chemi-cally cleaned and inspected with respect to particlepollution or scratches in the clean room After chamberexchange the load-lock system at the gun is evacuatedand vacuum backed Finally two Mg cathodes are lasercleaned QEs are measured and the best cathode is loadedinto the gun The QE stays stable for a very long timenot only in the transport chamber with a vacuum ofle10minus10 mbar but also during SRF gun operation itselfThe cathode has always a shiny silver color but thesurface is changed as shown in Fig 7 The virgin part isthe polished mirrorlike surface while the cleaned partshows a periodic structure created by the scanning patternof the laser beamUp to now the study of Mg photocathodes has con-

centrated on issues affecting a stable and reliable gunoperation for terahertz users ie high QE and longlifetime Nevertheless a detailed study of the thermalemittance of Mg is still in progress Surface photoemissionplays an important role for Mg and thus the thermalemittance is expected to be a factor of 2 lower (05 μm=mm[22]) than predicted by the three-step model for metalsassuming volume photoemission [26] On the other handthe surface roughness of 100 nm with a period of 3ndash20 μm

TABLE II Rms timing jitter of rf reference SRF gun rf fieldand laser oscillator measured using Rhode amp Schwarz FSWPphase noise analyzer

Range13 GHzreference SRF gun rf

Laseroscillator

1ndash10 Hz 752 fs 1013 fs 1606 fs10 Hzndash10 MHz 273 fs 231 fs 857 fs1 Hzndash10 MHz 800 fs 1039 fs 1821 fs

FIG 6 Geometry of photocathode The insert shows a Mg plugphotograph

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produced by the laser increases the thermal emittanceApplying eg the two-dimensional model by Qian [27]on these values a growth by Δεth frac14 067ndash026 μm for14 MV=m cathode field and 1 mm rms laser spot size isexpected

D Drive laser system

The drive laser system for the SRF gun II has beendeveloped by Max Born Institute Berlin The laser has twochannels intended for different pulse repetition rates Onechannel is for 13 MHz repetition rate another for the range10ndash500 kHz Both channels produce Gaussian-like pulsesin the time domain of 3 and 5 ps FWHM length at 13 MHzand in the kilohertz region respectively Average outputpower is up to 05 W at 262 nmThe schematic layout of the drive laser system is shown

in Fig 8 The common part of the laser consists of a Ndglass mode-locked oscillator (52 MHz) followed by apulse picker that picks every fourth pulse thus reducing therepetition rate to 13 MHz Then the beam goes into either afiber amplifier or a regenerative amplifier in case ofoperating at 13 MHz or at 10ndash500 kHz respectively

The final amplification is provided by a multipass amplifierwhich is common for both channels At the end the systemsplits again into two frequency conversion modules Eachmodule consists of a lithium triborate (LBO) and a betabarium borate (BBO) crystal that consequently provideIR-to-green and green-to-UV conversion Because of a bigdifference in pulse energy at 13 MHz and at 10ndash500 kHzdifferent focusing is required to achieve the right peakpower density within the crystalsIn practice it turned out that the IR-to-UV conversion is

likely to be the main source of instabilities in the outputlaser beamWhen the IR part is rather stable there are still afew degrees of freedom to play with to optimize theconversion If not optimized the crystals can be too muchaffected by thermal load from the laser beam and therebydegrade quickly andor be too sensitive to small variationsin the IR input and in the environment A significantimprovement in both long-term power stability and spatialbeam profile (in the kilohertz module) has been achievedrather recently by modifying the beam configuration andplacement of the crystals Figure 9 illustrates the progressin the spatial beam profile

FIG 7 Photograph of Mg photocathode Mg214 after second laser cleaning (left) and surface structure changes after cleaning (opticalmicroscope image right)

FIG 8 Scheme of the two-channel laser developed by MBI Berlin

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After the laser an attenuator consisting of a quarter-waveplate and a polarizer is the first optical device on the UVbeam path The attenuator is used to set the required laserpower on the photocathode and to compensate for occa-sional power variations of the laser output Then a fastoptical mechanical shutter allows one to operate in so-calledldquomacropulse moderdquo (used when guiding and optimizing theelectron beam) where macropulses are 02ndash40 ms in lengthand their repetition rate is 1ndash10 Hz For the acceleratorsafety the beam must be shut down within 2 ms after adangerous level of electron beam loss has been detectedOwing to about 01 ms switching time the optical mechani-cal shutter allows one to satisfy this conditionVarious photodiodes and power meters and an autocor-

relator from APE GmbH Berlin (recently updated toperform autocorrelation directly in UV as well) are usedfor diagnostics and constant monitoring of the laser beamThe most recent autocorrelation measurement at UVconfirmed the pulse length of 23 ps rms (53 ps FWHM)The laser beam transport from the laser table to the

photocathode has a length of about 135 m A physicalaperture placed at the end of the optical table is imaged ontothe cathode with a magnification of 25 by two lensesAttention should be paid to using lenses and other

transmitting elements in a pulsed UV laser beam We haveencountered an effect when UV-grade fused silica experi-ences UV-induced compaction [28] becomes birefringentand thus progressively affects the beam profile There aredifferent grades of UV fused silica on the market and wehave observed that their resilience to UV laser radiationmay differ significantly At present we simply avoid usingstandard UV fused silica elements close to the laser outputThe first lens is placed 2 m after the beam-shaping aperturewhere the beam is attenuated and rather wide due to somegeometrical divergence which it has at the output of the UVconversion and thus also after the aperture Under theseconditions we have not observed any degradation of lensesthat perform imaging of the aperture on the cathode Thereis a row of apertures of different diameters placed on amotorized linear stage so that one can change between themremotely A motorized mirror allows one to finely positionthe laser spot on the cathode About 2 m before the cathodean uncoated wedge picks up about 5 of laser power forthe so-called virtual cathode The virtual cathode is a Ce-doped yttrium aluminum garnet (YAGCe) screen placed atthe same distance from the wedge as the real cathode so thatit allows one to see how the spot on the real cathode looksand how it moves over the cathode with 1∶1 ratio TheYAGCe screen is imaged on a CCD camera and fluorescesat green when being irradiated by the UV light

E Diagnostic beam line and connectionto ELBE linac

A diagnostics beam line is directly connected to the SRFgun [29] (see Fig 10) whereby its first part also serves forthe beam transport into the ELBE accelerator After thelaser input port a first screen station with a retractableFaraday cup is situated Following the beam direction threequadrupoles and a 45deg bending magnet for the ELBEconnection are installed In a straight direction the secondscreen station additionally contains a moving slit system fortransverse phase space and emittance measurements Aftera third screen station a 180deg spectrometer magnet is placed

FIG 9 Laser spot profile on the cathode before and afteroptimization of the UV conversion (the spot is 38 mm indiameter and is formed as an optical image of a physical apertureon the optical table)

FIG 10 The diagnostic beamline of the ELBE SRF gun II

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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produced by the laser increases the thermal emittanceApplying eg the two-dimensional model by Qian [27]on these values a growth by Δεth frac14 067ndash026 μm for14 MV=m cathode field and 1 mm rms laser spot size isexpected

D Drive laser system

The drive laser system for the SRF gun II has beendeveloped by Max Born Institute Berlin The laser has twochannels intended for different pulse repetition rates Onechannel is for 13 MHz repetition rate another for the range10ndash500 kHz Both channels produce Gaussian-like pulsesin the time domain of 3 and 5 ps FWHM length at 13 MHzand in the kilohertz region respectively Average outputpower is up to 05 W at 262 nmThe schematic layout of the drive laser system is shown

in Fig 8 The common part of the laser consists of a Ndglass mode-locked oscillator (52 MHz) followed by apulse picker that picks every fourth pulse thus reducing therepetition rate to 13 MHz Then the beam goes into either afiber amplifier or a regenerative amplifier in case ofoperating at 13 MHz or at 10ndash500 kHz respectively

The final amplification is provided by a multipass amplifierwhich is common for both channels At the end the systemsplits again into two frequency conversion modules Eachmodule consists of a lithium triborate (LBO) and a betabarium borate (BBO) crystal that consequently provideIR-to-green and green-to-UV conversion Because of a bigdifference in pulse energy at 13 MHz and at 10ndash500 kHzdifferent focusing is required to achieve the right peakpower density within the crystalsIn practice it turned out that the IR-to-UV conversion is

likely to be the main source of instabilities in the outputlaser beamWhen the IR part is rather stable there are still afew degrees of freedom to play with to optimize theconversion If not optimized the crystals can be too muchaffected by thermal load from the laser beam and therebydegrade quickly andor be too sensitive to small variationsin the IR input and in the environment A significantimprovement in both long-term power stability and spatialbeam profile (in the kilohertz module) has been achievedrather recently by modifying the beam configuration andplacement of the crystals Figure 9 illustrates the progressin the spatial beam profile

FIG 7 Photograph of Mg photocathode Mg214 after second laser cleaning (left) and surface structure changes after cleaning (opticalmicroscope image right)

FIG 8 Scheme of the two-channel laser developed by MBI Berlin

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After the laser an attenuator consisting of a quarter-waveplate and a polarizer is the first optical device on the UVbeam path The attenuator is used to set the required laserpower on the photocathode and to compensate for occa-sional power variations of the laser output Then a fastoptical mechanical shutter allows one to operate in so-calledldquomacropulse moderdquo (used when guiding and optimizing theelectron beam) where macropulses are 02ndash40 ms in lengthand their repetition rate is 1ndash10 Hz For the acceleratorsafety the beam must be shut down within 2 ms after adangerous level of electron beam loss has been detectedOwing to about 01 ms switching time the optical mechani-cal shutter allows one to satisfy this conditionVarious photodiodes and power meters and an autocor-

relator from APE GmbH Berlin (recently updated toperform autocorrelation directly in UV as well) are usedfor diagnostics and constant monitoring of the laser beamThe most recent autocorrelation measurement at UVconfirmed the pulse length of 23 ps rms (53 ps FWHM)The laser beam transport from the laser table to the

photocathode has a length of about 135 m A physicalaperture placed at the end of the optical table is imaged ontothe cathode with a magnification of 25 by two lensesAttention should be paid to using lenses and other

transmitting elements in a pulsed UV laser beam We haveencountered an effect when UV-grade fused silica experi-ences UV-induced compaction [28] becomes birefringentand thus progressively affects the beam profile There aredifferent grades of UV fused silica on the market and wehave observed that their resilience to UV laser radiationmay differ significantly At present we simply avoid usingstandard UV fused silica elements close to the laser outputThe first lens is placed 2 m after the beam-shaping aperturewhere the beam is attenuated and rather wide due to somegeometrical divergence which it has at the output of the UVconversion and thus also after the aperture Under theseconditions we have not observed any degradation of lensesthat perform imaging of the aperture on the cathode Thereis a row of apertures of different diameters placed on amotorized linear stage so that one can change between themremotely A motorized mirror allows one to finely positionthe laser spot on the cathode About 2 m before the cathodean uncoated wedge picks up about 5 of laser power forthe so-called virtual cathode The virtual cathode is a Ce-doped yttrium aluminum garnet (YAGCe) screen placed atthe same distance from the wedge as the real cathode so thatit allows one to see how the spot on the real cathode looksand how it moves over the cathode with 1∶1 ratio TheYAGCe screen is imaged on a CCD camera and fluorescesat green when being irradiated by the UV light

E Diagnostic beam line and connectionto ELBE linac

A diagnostics beam line is directly connected to the SRFgun [29] (see Fig 10) whereby its first part also serves forthe beam transport into the ELBE accelerator After thelaser input port a first screen station with a retractableFaraday cup is situated Following the beam direction threequadrupoles and a 45deg bending magnet for the ELBEconnection are installed In a straight direction the secondscreen station additionally contains a moving slit system fortransverse phase space and emittance measurements Aftera third screen station a 180deg spectrometer magnet is placed

FIG 9 Laser spot profile on the cathode before and afteroptimization of the UV conversion (the spot is 38 mm indiameter and is formed as an optical image of a physical apertureon the optical table)

FIG 10 The diagnostic beamline of the ELBE SRF gun II

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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After the laser an attenuator consisting of a quarter-waveplate and a polarizer is the first optical device on the UVbeam path The attenuator is used to set the required laserpower on the photocathode and to compensate for occa-sional power variations of the laser output Then a fastoptical mechanical shutter allows one to operate in so-calledldquomacropulse moderdquo (used when guiding and optimizing theelectron beam) where macropulses are 02ndash40 ms in lengthand their repetition rate is 1ndash10 Hz For the acceleratorsafety the beam must be shut down within 2 ms after adangerous level of electron beam loss has been detectedOwing to about 01 ms switching time the optical mechani-cal shutter allows one to satisfy this conditionVarious photodiodes and power meters and an autocor-

relator from APE GmbH Berlin (recently updated toperform autocorrelation directly in UV as well) are usedfor diagnostics and constant monitoring of the laser beamThe most recent autocorrelation measurement at UVconfirmed the pulse length of 23 ps rms (53 ps FWHM)The laser beam transport from the laser table to the

photocathode has a length of about 135 m A physicalaperture placed at the end of the optical table is imaged ontothe cathode with a magnification of 25 by two lensesAttention should be paid to using lenses and other

transmitting elements in a pulsed UV laser beam We haveencountered an effect when UV-grade fused silica experi-ences UV-induced compaction [28] becomes birefringentand thus progressively affects the beam profile There aredifferent grades of UV fused silica on the market and wehave observed that their resilience to UV laser radiationmay differ significantly At present we simply avoid usingstandard UV fused silica elements close to the laser outputThe first lens is placed 2 m after the beam-shaping aperturewhere the beam is attenuated and rather wide due to somegeometrical divergence which it has at the output of the UVconversion and thus also after the aperture Under theseconditions we have not observed any degradation of lensesthat perform imaging of the aperture on the cathode Thereis a row of apertures of different diameters placed on amotorized linear stage so that one can change between themremotely A motorized mirror allows one to finely positionthe laser spot on the cathode About 2 m before the cathodean uncoated wedge picks up about 5 of laser power forthe so-called virtual cathode The virtual cathode is a Ce-doped yttrium aluminum garnet (YAGCe) screen placed atthe same distance from the wedge as the real cathode so thatit allows one to see how the spot on the real cathode looksand how it moves over the cathode with 1∶1 ratio TheYAGCe screen is imaged on a CCD camera and fluorescesat green when being irradiated by the UV light

E Diagnostic beam line and connectionto ELBE linac

A diagnostics beam line is directly connected to the SRFgun [29] (see Fig 10) whereby its first part also serves forthe beam transport into the ELBE accelerator After thelaser input port a first screen station with a retractableFaraday cup is situated Following the beam direction threequadrupoles and a 45deg bending magnet for the ELBEconnection are installed In a straight direction the secondscreen station additionally contains a moving slit system fortransverse phase space and emittance measurements Aftera third screen station a 180deg spectrometer magnet is placed

FIG 9 Laser spot profile on the cathode before and afteroptimization of the UV conversion (the spot is 38 mm indiameter and is formed as an optical image of a physical apertureon the optical table)

FIG 10 The diagnostic beamline of the ELBE SRF gun II

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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followed by two screen stations in the straight and 180degdirections respectively All screen stations are equippedwith YAGCe screens Properly distributed beam positionmonitors steering coils and charge monitors as well as ahigh-power beam dump at the straight end complete thebeam lineIn order to guide the beam to the ELBE main accelerator

an achromatic dogleg structure was built consisting of thedipole in the diagnostic beam line a second dipole magnetand a second quadrupole triplet between these dipolesThereby the first quadrupole triplet serves to match thebeam to the dogleg and the second triplet in the doglegensures achromatic beam transport

III ELECTRON GUN BEAM PARAMETERS

The purpose of SRF gun II is to generate a cw beam withhigher energy and higher bunch charge but with shorterbunch length and lower transverse emittance than theroutinely used dc gun with thermionic cathode at ELBETable III summarizes the main beam parameters of SRFgun II Since 2015 the gun is able to deliver a 4-MeVelectron beam with a bunch charge up to 300 pC into thediagnostic beam line and up to 250 pC into ELBEMeasurements and gun parameter optimization were

accompanied by detailed beam dynamical simulations[30] The charge emission of the photocathode and thebeam dynamics in the SRF gun were simulated with ASTRA

[31] and further beam transport was computed usingELEGANT [32] in which coherent synchrotron radiationin the bending magnets and the longitudinal space chargeeffects are considered These codes were also utilized todevelop and optimize the beam transport into the ELBEaccelerator for the terahertz radiation production [33] asdiscussed later in Sec IVThe main setting parameters of the gun are the drive laser

parameters which are the pulse length of the temporalGaussian shaped laser the laser spot size and its distribu-tion at the cathode the laser pulse energy and the laserphase (bunch launches with respect to rf phase) Furtherparameters are the rf acceleration gradient the cathode dc

bias the cathode retraction position and the solenoidcurrentOne of the most significant parameters is the laser phase

Its calibration needs a regular check and correction basedon a laser phase scan at low bunch charge (fixed laserpower) as presented in Fig 11 Defining the rf field assinuslike in time the beam onset is at 0deg with a strongincrease With growing phase and corresponding extractionfield the Schottky effect reduces the effective workfunction and thus QE and beam current increase Theabrupt drop at about 105deg is due to phase mismatch of theelectrons when they reach the TESLA cells of the cavityThe additional field of the negative cathode bias shifts thebeam onset in the curve to the left For higher bunchcharges the photoemission in the SRF gun is dominated bythe Schottky effect and space charge In order to demon-strate the space charge effect curves are measured withstepwise increased laser power Thereby this effect domi-nates more and more especially for lower laser phaseswhere the launch field is small Charges can be extracted upto about 300 pCEnergy and energy spread of the beam were measured

with the 180deg dipole magnet in the diagnostics beam line(see Fig 10) The arrangement of screen 4 in the forwarddirection downstream from the dipole and screen 5 in the180deg direction allows a beam spot size correction of thedispersive beam profile measured on screen 5 and thus amore accurate energy spread measurement The kineticenergy for various acceleration gradients is presented inFig 12 Measurements were performed up to 9 MV=m Atthe standard operation gradient for beam production toELBE of 8 MV=m the kinetic energy is about 4 MeV Thismaximum of beam energy is at a launch phase of about 55degand the corresponding acceleration field strength at thecathode is then 119 MV=m For terahertz radiation

TABLE III Main beam parameters of SRF gun II with Mgcathode (gun gradient 8 MV=m)

Parameter Simulated [30] Measured

Kinetic energy (MeV) 4 4Bunch charge (pC) 200 0ndash300Transverse emittancerms (mmmrad)

58a 2ndash15

Energy spread rms (keV) 98a 5ndash25Micropulse rate (kHz) 10ndash500Beam current in cw (μA) le30Dark current (nA) 40

aAt 200 pC bunch charge 65deg laser phase and 4 mm diameterlaser spot

FIG 11 Measured Faraday cup current and correspondingbunch charge versus drive laser phase Curves are shown forvarious UV laser powers

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

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the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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production the gun is operated at lower launch phase ofabout 45deg and with accordingly 103 MV=m launch fieldThe phase of the maximum energy for different gradients

increases with the gradient and results from the half-celldesign length ie the traveling time in it and thecorresponding arrival phase at the TESLA cells Thisfeature also determines the energy spread behavior aspresented in Fig 13 At laser phases lower than the valuefor maximum energy the electron bunches have a negativeenergy correlation (head has lower energy) and at higherlaser phases the energy correlation is positive Near thepoint of maximum energy the rms energy spread has itsminimum but it is increasing with the bunch charge

The transverse emittance measurements were carried outusing a quadrupole scan and slit-mask scan technique Avertically moving slit is installed in screen station 2 and theimages can be measured with screen station 3 (see Fig 10)Results of a series of slit-scan measurements for differentbunch charges are presented in Fig 14 for a laser spot of25 mm diameter and with a Gaussian-shaped laser pulse of23 ps rms In general there are three contributions to thetransverse emittance thermal emittance of the photoca-thode field effects of the cavity rf field and the solenoidfield and space charge effects These effects are alsoincluded in the ASTRA simulation presented for compari-son The gun and the corresponding simulation parameterslike laser pulse length laser spot size and laser phase arechosen in order to obtain a minimum bunch length of theproduced bunch for 200 pC which is the most importantcriteria for terahertz production application described inSec IV For that reason the transverse emittance values arerelatively high in simulation and in measurement Thedifferences between experimental and simulation valuescan be caused by a number of effects First as discussedabove the higher thermal emittance due to the cathoderoughness is not included in the simulation and couldexplain the higher emittance at low bunch charges Secondthe simulation was performed for a perfect flattop laserprofile but the experimental transversal laser profile at thecathode was very inhomogeneous and asymmetric in themeasurement in year 2017 (see Fig 9) A similar effectcould also come from the QE inhomogeneity of thephotocathodes But as visible in the 2020 results theimprovement of the laser profile results in a good agree-ment with the simulation results Finally we observedastigmatism which may be caused by the asymmetry of therf field due to the coupler and imperfection or misalignment

FIG 12 Kinetic energy at gun exit versus laser phase foracceleration gradients between 6 and 9 MV=m

FIG 13 Energy spread as a function of the laser phase fordifferent bunch charges measured with the 180deg bending magnetof the diagnostics beam line

FIG 14 Measured normalized transverse emittance with slitscan versus bunch charge and comparison with ASTRA simulationresults

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

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compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

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gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-14

the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

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of the solenoid These effects have been partly compen-sated now with quadrupole correctors similar to the designproposed in Ref [34] and used at the photo injector testfacility (PITZ) photoinjectorAt present there is no bunch length measurement

method available in the diagnostic beam line But byguiding the beam via the dogleg to the first linac module ofELBE a phase variation of the first cavity and a followingdipole can be utilized for a longitudinal phase spacemeasurement This phase scan method is described inRefs [3536] and is quasi the longitudinal analogon ofthe quadrupole scan Since it samples the beam at the firstcavity usually a correction is needed to obtain the values atthe position of electron gun exit However here we aredirectly interested in the measurement values at entrance oflinac module 1 because the later bunch compressionrequires bunches as short as possible at this positionFigure 15 presents the measurement results for bunchesof 3 and 200 pC It can be seen that the negativelongitudinal correlation that the bunch has at small laserphases leads to bunch compression (velocity bunching) forlow charges and preserves the initial bunch length for200 pC bunches (laser with 23 ps rms Gaussian profile)

IV SRF GUN APPLICATION FOR TERAHERTZRADIATION PRODUCTION

A TELBE facility at HZDR

Terahertz (THz) radiation which is located between theinfrared light and microwaves in the frequency domainattracts much interest due to its wide applications inbiomedicine imaging and security communication andcondensed matter physics Intense THz sources are newtools to study nonlinear THz field-induced carrier dynamics

in condensed matter like photoexcitation intense THz-pump and THz-probe experiments THz-induced electronfield emission switching of superconductors magnetic andpolarization switching or nonlinear response of metama-terials [3738] In accelerator-based radiation sources theuse of relativistic electrons and the use of superradianceie ultrashort electron bunches (tenths of picoseconds) inwhich electrons radiate in phase have been the keyelements The TELBE facility at the ELBE radiation sourcewas established in 2013 [739] and started user operation in2016 TELBE combines superradiant THz radiationsources with a superconducting accelerator operating incw mode and with pulse repetition rates up to severalmegahertz Therefore a high brilliant THz source with highaverage power high pulse energy and high flexibility inwavelength tuning and in pulse time structure can beoffered to users TELBE has two radiation stages whichcan be simultaneously used One generates broadbandsingle-cycle coherent diffraction radiation (CDR) and thesecond one tunable small bandwidth coherent undulatorradiation (CUR) Both kinds of radiation are transported toa THz user lab where the radiation can be characterizedand applied for a variety of experimental setups Theradiation is created by relativistic subpicosecond electronbunches For coherent radiation that means the longi-tudinal bunch length is significantly shorter than thewavelength and the intensity grows with the square ofthe number of electrons Therefore the spectral angulardistribution of the radiation of an electron bunch can bewritten as

d2IdωdΩ

frac14 Nf1thorn ethN minus 1THORNFethωTHORNg d2IsedωdΩ

eth1THORN

Here N denotes the number of electrons in the bunchω is the radiation angular frequency and d2Ise=dωdΩ is thespectral angular intensity for a single electron The bunchlongitudinal form factor FethωTHORN of the normalized chargedistribution ρethzTHORN is given by

FethωTHORN frac14

Z

ρethzTHORNeminusiωz=cdz

2

eth2THORN

and has a value between zero and one As can be seen inEq (1) the spectral power splits into an incoherent and acoherent part For the coherent part the quadratic increasewith bunch charge sustains as long as the form factor doesnot decrease Consequently a higher bunch charge needssuccessful bunch compression

B Beamline and beam transport

The gun acceleration gradient was chosen to be80 MV=m which is the maximum value to hold fieldemission helium consumption and dark current to accept-able levels and it also ensures a stable operation of the gun

FIG 15 Bunch length versus laser phase for 3 and 200 pC atthe entrance of accelerator module 1 (cavity C1) measured withthe cavity phase scan method

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With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-12

compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

SUCCESSFUL USER OPERATION OF A hellip PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-13

gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-14

the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

SUCCESSFUL USER OPERATION OF A hellip PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-15

With this setting the rf of the gun runs without disruptionsfor typical user beam times of one week or even longer TheTHz application requires bunch charges as high as possibleand short electron bunch lengths already at the gun exit aswill be discussed later The transverse emittance plays aless important role Parameters forming a good compro-mise for THz production are a 23 ps rms length of theGaussian-shaped laser pulse a laser spot size of 4 mmdiameter at the cathode and bunch charges of 200 pC andrecently of 250 pC The photocathode was retracted by13 mm and had a bias of minus5 kV The standard workingpoint of the gun is at a laser phase of about 50deg which ischaracterized by the maximum of electron energy and theminimum of rms energy spread The latter is important forthe following beam transport through the dispersive doglegsection However for THz production the laser phase is setto a different value of about 45deg in order to obtain therequired shorter bunch length at gun exitFor THz production at ELBE the beam line layout with

the SRF gun dogleg the two accelerator modules mag-netic chicane undulator and the path to the beam dump isschematically shown in Fig 16 The SC solenoid and thefirst quadrupole triplet downstream from the SRF gun areset to match the beam for the dogleg The quadrupolesinside the dogleg ensure achromaticity In the first ELBElinac module (cavities C1 and C2) the electron bunches areaccelerated to 155 MeV with on-crest rf phases The

second ELBE linac module (cavities C3 and C4) serves forfurther acceleration as well as to create the energy chirp byoff-crest operation The final beam energy of about 26 MeVis held constant ie any rf phase variations of C3 and C4are compensated by corresponding changes of the gra-dients The energy chirp ie a correlation between particlemomentum and longitudinal coordinate in the bunch isneeded for the following magnetic bunch compression inthe chicane The chicane consists of four parallel facedipole magnets After the chicane the first THz radiationsource is situated which produces CDR and consists of ascreen with aperture of 4 mm diameter The followingsecond THz source can produce CUR in the spectral rangeof 01ndash3 THz The electromagnetic undulator has eightperiods with a period length of 300 mm

C Electron beam properties

The following discussion is focused on the longitudinalproperties of the beam since the final bunch length at theradiation stations determines the coherence and thus theoutput power of THz radiation Figure 17 presents simu-lation results for the evolution of the bunch length σt alongthe beam line The curves are for three different laserphases Beside the laser pulse length itself the laser phasehas a large effect on the bunch length at the entrance oflinac 1 This phase determines whether the bunch is

FIG 16 Beamline layout for THz radiation production at ELBE

FIG 17 Evolution of rms bunch length in the beam line from SRF gun to THz radiator for three different SRF gun drive laser phasesThe simulation results are obtained from ASTRA and ELEGANTwith typical experimental gun parameters 205 MV=m gun peak field onaxis 4 mm laser spot diameter 23 ps rms laser pulse length and 200 pC bunch charge

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-12

compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

SUCCESSFUL USER OPERATION OF A hellip PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-13

gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-14

the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

SUCCESSFUL USER OPERATION OF A hellip PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-15

compressed (velocity bunching) or not within the gun andin the dogleg section which means the bunch lengths are28 ps for 35deg and 48 ps for 50deg laser phase As aconsequence of shorter bunch lengths there are decreasingnonlinear energy spread contributions from linac 1 andsmaller correlated energy spread values at optimum chirpphase after linac 2 In Fig 18 the simulation results of thelongitudinal phase space after compression at the THzradiator position are presented The nonlinear energyspread produced by linac 1 gives the C-shape distributionsin the cases of the 50deg and 45deg laser phases These shapes

define the final bunch length Only for the 35deg case is thebunch short enough at linac 1 that this shape is not visibleand the remaining uncorrelated energy spread determinesthe final bunch lengthExperimentally laser phases between 40deg to 45deg were

applied in most of the cases for which sufficiently com-pressed electron pulses could be produced The compressedelectron bunch length was measured with a Martin-Puplettinterferometer [40] analyzing coherent transition radiationin front of the undulator Bunch length results as a functionof compression (C4 phase) for different bunch charges and

FIG 18 Simulation results of longitudinal phase space distribution for 35deg laser phase (a) 45deg laser phase (b) and 50deg laser phase (c)at the THz radiator position The associated longitudinal charge distributions are shown in the plots (d) to (f)

FIG 19 Bunch length measurement after bunch compression with magnetic chicane versus cavity C4 phase for different bunchcharges (a) and different drive laser phases (b)

SUCCESSFUL USER OPERATION OF A hellip PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-13

gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-14

the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

SUCCESSFUL USER OPERATION OF A hellip PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-15

gun drive laser phases are presented in Fig 19 In agree-ment with the simulations these measurements show that alower laser phase results in shorter compressed bunches

D Results of THz radiation production

The measured THz power as a function of bunch chargeis shown in Fig 20 The bunch charge is varied by changingthe photocathode laser pulse energy Up to 150 pC a clearquadratic dependency is visible whereas for higher chargesthe behavior is more linear The reason is an increase inbunch length but it also needs a more careful optimizationof beam transport and compression at each charge valuedue to increasing space charge effects which was not donein this measurement For compression the chicane was seton a fixed value in order to sustain the transverse beam

parameters Then the phases of C3 and C4 were tuned foroptimal bunching together with the gradients holding thefinal beam energy at a constant value Fine-tuning was donewith the phase of cavity C4 Figure 21 shows the normal-ized intensity histogram of the THz intensity over a typicalmeasurement duration of 10 min This measurement con-tains data points from 75 times 106 THz pulses (repetition rateof 50 kHz) The data were obtained using a fast pyroelectricdetector which transforms the incident THz radiation into avoltage signal that is read out using a fast analog-to-digitalconverter Before impinging on the detector the narrow-band THz radiation from the undulator source propagatesthrough a THz bandpass filter with a central frequency of07 THz and a bandwidth of 20 (FWHM) The distribu-tion of THz intensities is clearly Gaussian with a standarddeviation of about 003 demonstrating the high-intensitystability of the THz sourceFigure 22 presents the cw THz power within a frequency

range of 015ndash15 THz produced during user shifts with 50and 100 kHz pulse repetition rate in the years 2018 and2019 Typical power values for 200 pC bunch charge arebetween 300 and 500 mW The maximum values are for250 pC charge with 1 W at 03 THz (10 μJ) and 620 mWwith 50 kHz at 07 THz (124 μJ) Before 2018 thethermionic injector had been used for THz production atELBE In comparison the THz output applying the SRFgun could be increased up to a factor of 5 for 200 pC and bya factor of 10 for 250 pC bunch charge

V CONCLUSIONS

As an RampD effort to explore a new cw injector for theELBE accelerator SRF gun II is actually a successfulprototype and an excellent demonstration of SRF guntechnology for user operation With this new injector

FIG 20 Dependence of undulator radiation power at 07 THzon electron bunch charge

080 085 090 095 100 105 110 115 1200

2x105

4x105

6x105

8x105

1x106

num

ber

of p

ulse

s

Normalized THz intensity (arb units)

07 THz tune after 07 THz bandpassfilter with 20 bandwidth

Intensity histogramm Gaussian distribution fit

FIG 21 Normalized THz intensity histogram for a typical10 min measurement The standard deviation of the Gaussiandistribution is 00328

FIG 22 Undulator radiation power produced in cw operationfor different frequencies at the TELBE facility

J TEICHERT et al PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-14

the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

[17] A Arnold P Murcek J Teichert R Xiang P Kneisel LTurlington M Stirbet and G Eremeev Fabricationtuning treatment and testing of two 35 cell photo-injectorcavities for the ELBE linac in Proceedings of the 15thInternational Conference on RF Superconductivity(SRF2011) Chicago IL (JACoW Geneva 2011) p 405

[18] A Arnold M Freitag P Murcek J Teichert H Ven-nekate R Xiang P Kneisel G Ciovati and L TurlingtonRF performance results of the 2nd ELBE srf gun inProceedings of the 17th International Conference onSuperconductivity (SRF 2015) Whistler Canada (JACoWGeneva 2015) p 1227

[19] A Arnold M Freitag P Murcek J Teichert H VennekateR Xiang P Kneisel G Ciovati and L TurlingtonCommissioning results of the 2nd 35 cell SRF gun forELBE in Proceedings of the 27th Linear AcceleratorConference (LINAC2014) Geneva Switzerland (JACoWGeneva 2014) p 578

[20] E T Tulu U van Rienen and A Arnold Differentcountermeasures of electron amplification in the photo-cathode unit in Proceedings of the 5th InternationalParticle Accelerator Conference (IPAC2014) DresdenGermany (JACoW Geneva 2014) p 652

[21] R Xiang A Arnold T Kamps P Lu P Michel PMurcek H Vennekate G Staats and J Teichert Exper-imental studies of dark current in a superconducting rfphotoinjector Phys Rev Accel Beams 17 043401 (2014)

SUCCESSFUL USER OPERATION OF A hellip PHYS REV ACCEL BEAMS 24 033401 (2021)

033401-15

the ELBE center is able to provide ultrashort electronbunches with significantly higher charge to the THz facilitythan delivered by the thermionic dc gunUntil now (April 2020) the SRF gun has been applied

for the ELBE accelerator in 119 THz shifts and 18 shifts forother purposes ie for about 1640 h beam time Duringthese months two Mg photocathodes were operated in theSRF gun and delivered a total charge of 506 C Theadvantages of Mg photocathodes consisting of solid Mgplugs are easy fabrication a relatively high QE after lasercleaning and a low risk of surface contamination of thesuperconducting cavity And neither a degradation of cavityperformance nor an increase of field emission was observedduring the operation with Mg photocathodes since 2016

ACKNOWLEDGMENTS

We thank the whole ELBE team for their help with thisproject The work was partly supported by the GermanFederal Ministry of Education and Research (BMBF) GrantNo 05K12CR1 and Deutsche Forschungsgemeinschaft(DFG) project (XI 1062-1)

[1] P Michel ELBE Center for high-power radiation sourcesJ Large-Scale Res Facil 2 A39 (2016)

[2] C Hernandez-Garcia D Bullard F Hannon Y Wang andM Poelker High voltage performance of a dc photoemis-sion electron gun with centrifugal barrel-polished electro-des Rev Sci Instrum 88 093303 (2017)

[3] M Yamamoto and N Nishimori High voltage thresholdfor stable operation in a dc electron gun Appl Phys Lett109 014103 (2016)

[4] A Arnold and J Teichert Overview on superconductingphotoinjectors Phys Rev Accel Beams 14 024801(2011)

[5] F Sannibale D Filippetto H Qian C Mitchell F ZhouT Vecchione R K Li S Gierman and J SchmergeHigh-brightness beam tests of the very high frequency gunat the Advanced Photo-injector EXperiment test facility atthe Lawrence Berkeley National Laboratory Rev SciInstrum 90 033304 (2019)

[6] J Klug et al Development of a neutron time-of-flightsource at the ELBE accelerator Nucl Instrum MethodsPhys Res Sect A 577 641 (2007)

[7] B Green et al High-field high-repetition-rate sources forthe coherent THz control of matter Sci Rep 6 22256(2016)

[8] A Jochmann et al High Resolution Energy-AngleCorrelation Measurement of Hard X Rays from Laser-Thomson Backscattering Phys Rev Lett 111 114803(2013)

[9] A Arnold et al Development of a superconducting radiofrequency photoelectron injector Nucl Instrum MethodsPhys Res Sect A 577 440 (2007)

[10] J Teichert et al Free-electron laser operation with asuperconducting radio-frequency photoinjector at ELBE

Nucl Instrum Methods Phys Res Sect A 743 114(2014)

[11] X J Wang and P Musumeci Report of the Basic EnergySciences Workshop on the Future of Electron SourcesSLAC National Accelerator Laboratory 2016

[12] T Kamps bERLinPro project team SRF gun developmentfor energy recovery linac applications in High-BrightnessSources and Light-Driven Interactions OSA technicaldigest (Optical Society of America Washington DC2016) paper ET1A3 See also M Abo-Bakr et alProgress report of the Berlin energy recovery projectbERLinPro in Proceedings of the 2015 InternationalParticle Accelerator Conference Richmond VA (JACoWGeneva 2015) p 1438

[13] E Vogel et al SRF gun development at DESY inProceedings of the 29th Linear Accelerator ConferenceLINAC2018 Beijing China (JACoW Geneva 2015)p 105

[14] L W Feng et al Ultrafast electron diffraction withmegahertz MeV electron pulses from a superconductingradio-frequency photoinjector Appl Phys Lett 107224101 (2015)

[15] H Vennekate A Arnold P Lu P Murcek J Teichert andR Xiang Emittance compensation schemes for a super-conducting rf injector Phys Rev Accel Beams 21093403 (2018)

[16] P Murcek A Arnold H Buumlttig D Janssen M Justus PMichel G Staats J Teichert R Xiang and P KneiselModified SRF photoinjector for the ELBE at HZDR inProceedings of the 15th International Conference on RFSuperconductivity (SRF2011) Chicago IL (JACoWGeneva 2011) p 39

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