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Laser ionized preformed plasma at FACET
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2014 Plasma Phys. Control. Fusion 56 084011
(http://iopscience.iop.org/0741-3335/56/8/084011)
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Plasma Physics and Controlled Fusion
Plasma Phys. Control. Fusion 56 (2014) 084011 (7pp) doi:10.1088/0741-3335/56/8/084011
Laser ionized preformed plasma at FACET
S Z Green1, E Adli1,3, C I Clarke1, S Corde1, S A Edstrom1, A S Fisher1,J Frederico1, J C Frisch1, S Gessner1, S Gilevich1, P Hering1, M J Hogan1,R K Jobe1, M Litos1, J E May1, D R Walz1, V Yakimenko1, C E Clayton2,C Joshi2, K A Marsh2, N Vafaei-Najafabadi2 and P Muggli4
1 SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA2 Electrical Engineering Department, University of California Los Angeles, CA 90095, USA3 Department of Physics, University of Oslo, 0316 Oslo, Norway4 Max Planck Institute for Physics, Munich, Germany
E-mail: [email protected]
Received 13 January 2014, revised 15 May 2014Accepted for publication 4 June 2014Published 22 July 2014
AbstractThe Facility for Advanced Accelerator and Experimental Tests (FACET) at SLAC installed a10-TW Ti : sapphire laser system for pre-ionized plasma wakefield acceleration experiments.High energy (500 mJ), short (50 fs) pulses of 800 nm laser light at 1 Hz are used at the FACETexperimental area to produce a plasma column. The laser pulses are stretched to 250 fs beforeinjection into a vapor cell, where the laser is focused by an axicon lens to form a plasmacolumn that can be sustained over the desired radius and length. A 20 GeV electron bunchinteracts with this preformed plasma to generate a non-linear wakefield, thus accelerating atrailing witness bunch with gradients on the order of several GV m−1. The experimental setupand the methods for producing the pre-ionized plasma for plasma wakefield accelerationexperiments performed at FACET are described.
Keywords: plasma wakefield accelerator, laser ionization, plasma source
(Some figures may appear in colour only in the online journal)
1. Introduction
The Facility for Advanced Accelerator and Experimental Tests(FACET) at the SLAC National Accelerator Laboratory hasoperated as a National User Facility since 2011. It supports abroad user program in accelerator science, materials science,and other fields of research. The multi-GeV plasma wakefieldacceleration experiments (known as E-200) form the core of theFACET program to demonstrate a single-stage plasma-basedaccelerator for electrons and positrons. The main goal is todevelop a plasma module with beam parameters and energygain at the level required for novel radiation sources and futurelinear colliders [1]. Potential applications for plasma wakefieldaccelerators (PWFA) include free electron laser (FEL) energydoublers and plasma afterburners for a linear collider or a futureHiggs factory.
For next generation PWFA experiments, long, uniform,high-density (>1016 e− cm−3) plasmas are required to producea large energy gain in a single meter-scale module, on the orderof tens of GeV. Best performance is achieved when the plasma
radius is greater than the blow-out radius, which is on the orderof the plasma electron skin depth c/ωpe or about 17 µm for theabove density. In the SLAC linac, the FACET electron beam isaccelerated to 20 GeV with a charge of 3 nC. In E-200 PWFAexperiments, a plasma is formed in a heat-pipe oven [2] filledwith an alkali metal vapor (Li, Rb or Cs). Lithium is utilizedas the plasma source due to its relatively low first ionizationpotential (5.4 eV) and much higher second electron ionizationpotential (75.6 eV). As a result, the plasma density reachedthrough ionization is expected to be equal to the lithium atomicvapor density.
In a beam-driven PWFA, head erosion resulting from afinite beam emittance has been shown to be one of the limitingfactors in reaching the maximum energy gain [3]. The headof the electron beam is not guided until a plasma is formed;hence a preformed plasma will mitigate the head erosionproblem. With a pre-ionized plasma source, energy can beeffectively transferred from the electron beam to the wake.In the two-bunch PWFA experiment at FACET [4], a strongdrive beam produces an accelerating wake and a second trailing
0741-3335/14/084011+07$33.00 1 © 2014 IOP Publishing Ltd Printed in the UK
Plasma Phys. Control. Fusion 56 (2014) 084011 S Z Green et al
Figure 1. A diagram of the laser system.
witness beam follows the drive beam at the appropriate distancefor acceleration to high energy. With the two-bunch beamparameters at FACET, the drive beam cannot sufficiently ionizethe plasma, and even if it could, head erosion would limit thedistance over which acceleration can occur. Therefore, a laserionization scheme using axicon focusing has been developedto turn Li vapor into a preformed meter-scale homogenousplasma [5]. Discussion of the ionization process and the useof the axicon lens are detailed in [5]. The laser-ionized plasmaelectron density drops by a factor of two in approximately1.5 ns due to recombination, and thus needs to be synchronizedwith the arrival of the electron bunch and controlled to ∼100 psor better.
Single ionization of lithium demands maintaining a laserintensity larger than ∼1012 W cm−2 (based on an analyticalestimate of multiphoton ionization [5]) over the desired plasmaradius (∼1 mm) and length (∼1.5 m). The minimum laserenergy required depends on the ionization potential, densityand the volume of the vapor to be ionized. To make a 1.5 mlong lithium plasma with a radius of 1 mm and a density of∼5×1016 e− cm−3, the minimum laser energy is about 200 mJ,corresponding to a minimum peak power of 4 TW for a 50 fs(full-width at half-maximum, FWHM) laser pulse.
The E-200 experiments are the first to utilize the FACETlaser to generate a preformed plasma over 30 cm in lengthin June and December of 2013. Laser and axicon lens havebeen used to generate cm-scale plasmas in other experiments[6–8]. This paper describes plasma formation with the 10 TWFACET laser system and the setup for the first experiments, inwhich laser pulses were compressed to 250 fs, injected into avapor cell, and focused by an axicon lens to form the plasma.The interaction of the 20 GeV FACET electron beam with thispreformed plasma leads to acceleration in the wakefield withgradients on the order of a several GeV m−1.
2. The laser system
The 10 TW chirped pulse amplified Ti : Sapphire laser systemwas conceived, designed, installed and commissioned in lessthan seven months. The laser system consists of the front endlaser, preamplifier, main amplifier, 28 m-long laser transportline, compressor, and timing system, as described in the
following subsections. The 10 TW laser system enhancesthe FACET experimental program and enables a number ofexperiments utilizing a high-energy and/or high-peak-powerlaser with the FACET electron or positron beam.
2.1. The front end laser system, preamplifier, andmain amplifier
The laser oscillator is a Vitara-T from Coherent Inc., whichoperates at a center wavelength of 800 nm with a spectralbandwidth of 60 nm FWHM and a mode-locking rate of68 MHz. The oscillator is locked at the 8th harmonic of the476 MHz radiofrequency (RF) master reference of the SLAClinac, allowing control of the timing of the laser relative to theFACET beam. The oscillator is followed by a commercialregenerative amplifier (Regen) from Coherent Inc (LegendElite HE USP), where a portion of the laser is extracted priorto the Regen’s compressor to seed the amplifiers.
The Regen operates at 120 Hz, the maximum repetitionrate of the electron beam in the SLAC Linac, and is triggered bythe linac timing system. The Regen output laser repetition rateis divided down to 1 or 10 Hz to match the beam repetition ratedelivered to the FACET experimental area. One millijoule ofthe Regen’s uncompressed output energy is used for the poweramplifiers designed at SLAC. The first stage is an intermediatefour-pass preamplifier that delivers a 30 mJ seed to the mainamplifier. The preamplifier is pumped by a Quantel CFR200YAG flash lamp laser that can produce 130 mJ of 532 nm lightat 10 Hz. It uses an 8 mm diameter, 15 mm long Ti : Sapphirerod that is water cooled.
A four-pass main amplifier brings the energy up to ∼1 Jusing two Thales SAGA YAG flash lamp lasers that eachproduces 1.8 J of 532 nm light at 10 Hz. In this case, theTi : Sapphire crystal is a 20 mm-diameter, 20 mm-long water-cooled rod. The entire laser amplifier system fits onto twooptical tables located in the FACET laser room, at ground levelnext to the Klystron Gallery at Sector 20 of the SLAC Linac(2 km from the start of the 3 km linac), just above the FACETexperimental area. The laser system layout is illustrated infigure 1.
Two CCD cameras were set up to sample the laser profilesafter the main amplifier and before laser is transported out ofthe laser room. Figure 2 shows images of the near and far
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Plasma Phys. Control. Fusion 56 (2014) 084011 S Z Green et al
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Figure 2. Laser near and far field profiles after the main amplifier. The laser has flat-top profile with ∼25% intensity variation.
Figure 3. A block diagram for the laser locking system.
field profiles. These cameras are used to assess the laser modeintensity and phase profiles, as well as recover the launch vectorinto the transport.
2.2. Laser synchronization to the e-beam
The laser needs to be synchronized with the FACET e-beamfor PWFA experiments. A photodiode is used to time thelaser to the electron beam within 1 ns. The photodiode isplaced at a location on the beam line where both the lasersignal and optical transition radiation (OTR) from the electronbunch can be detected. The timing of the laser is adjusted toplace the laser pulse about 1 ns before the arrival of the electronbeam. The resolution is limited to 1 ns by the photodiode, longcables and the oscilloscope. Fine timing to ensure the e-beamarrives before the plasma density decays is done with theplasma acceleration signal itself. The laser timing is stepped in100 ps increments until there is no evidence of beam–plasmainteraction on downstream beam imaging diagnostics, whichindicates that the laser pulse arrives after the passage of electronbeam. From this point, the laser arrival time is stepped forwardin 10 ps increments until it is roughly 100 ps ahead of thee-beam.
The FACET laser is locked by an RF synchronization andphase stabilization system developed at SLAC. The timingsystem makes use of custom modular components designed
to fit into a simple off-the-shelf chassis, thereby enablingflexible controls system requirements. The locking systemconsists of an RF downmixer, a phase detector, a frequencymultiplier, a phase shifter and controller, a feedback unit and atrigger re-synchronizer, as illustrated in the block diagram offigure 3. The RF system locks the 56th harmonic (3808 MHz)of the 68 MHz mode-locked oscillator to the 8th harmonicof the 476 MHz accelerator master reference source. Thelaser oscillator pulse time relative to triggers is controlled byadjusting the cavity length of the Vitara oscillator via the phaseshifter to delay the oscillator relative to the accelerator RFsystem.
The laser jitter with respect to the RF reference has beenmeasured to 70 fs or better in a bandwidth from 10 Hz to10 kHz. Long-term drift over days has been estimated at<10 ps. The jitter between the laser and the electron beamis much larger (∼1 ps) than the jitter of the laser pulse arrival.In the 2 km of SLAC linac used to accelerate and compress theFACET electron bunch, phase and energy jitter result in arrivaltime variation of the electron beam on the order of <1 ps. Thisvalue is empirically determined by the laser delay needed totransition from no preformed plasma interaction to preformedplasma interaction on every shot. An electro-optic samplingsystem is currently being commissioned to quantify this jitteron a pulse-to-pulse basis.
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Figure 4. A top view of the laser transported to the experiment. The turning mirror and axicon lens are not drawn to scale. The dashed pathis used for laser diagnostics when the turning mirror is extracted to allow the laser pulses to be brought outside of the chamber for equivalentplane imaging.
2.3. Laser transport
The laser pulses are transported from the laser room to theFACET experimental area located inside the linac tunnel, 10 mbelow ground. The transport tubes pass through the wall fromthe laser lab to the Klystron Gallery, then along and acrossthe Klystron Gallery and down through a penetration to thelinac tunnel. To avoid beam quality degradation along thelong transport beam line due to the high peak power and thuspossible optics damage, the uncompressed laser is transportedthrough a system of five evacuated stainless steel tubes with adiameter of 6 inch (150 mm).
The main amplifier output plane is relay-imaged to theentrance plane of the compressor. There are two imagingstages: the magnification of the first stage is 3 : 1 and thatof the second is 1 : 1. Each stage of the imaging systemproduces a focus in the laser transport line. Therefore, it isessential to keep the transport tubes under vacuum to avoidair breakdown at the focal points which would cause lasershape and phase instabilities. The 28 m-long transport line isevacuated to a vacuum pressure of 10−5 Torr. A vacuum alsoavoids laser pointing instabilities from air convection drivenby uncontrolled temperature changes in the Klystron Gallery.The output of the imaging system preserves the collimatedinput laser qualities.
There are five dielectric turning mirrors in the transportline, and each is controlled remotely to align the laser. To aidthe alignment, a network camera is installed behind each of theturning mirrors. Each camera is placed outside the vacuum andallows observation through a viewport of the laser spot on a thinglass diffuser mounted directly behind the dielectric transportmirror.
2.4. Laser compressor
The laser pulses are compressed using a grating compressorin a vacuum chamber at the end of the transport line. Thecompressor chamber is adjacent to the FACET beamline,illustrated in figure 4.
The stretched pulses coming out of the main amplifier havea spectral bandwidth of 24 nm FWHM and can be compresseddown to less than 50 fs FWHM. The compressor is a two-grating compressor with a vertical retro-reflector. Because ofthe high peak power reached after compression (up to 10 TW),the compressor chamber has to be under vacuum. To avoidself-phase modulation when passing through the optics in theinteraction point (IP) chamber, the compressor (separationof the gratings) is detuned to increase the pulse length to∼250 fs. At the output of the chamber, an anti-reflective coatedwindow isolates the vacuum between the compressor and theFACET beam line. The total transmission efficiency of thelaser transport line and compressor has been measured to beabout 60%.
2.5. Integration of particle and laser beams
The laser pulses leaving the compressor are sent into a chamberin the FACET beam line. Figure 4 shows a top view of thebeam path at the location of the PWFA experiment and the twooptics inside this chamber. The compressed laser pulses passthrough a 2 inch (50 mm) diameter, 1.5◦ fused silica axiconlens with a mask of 3/8 inch (9.5 mm) diameter. The turningmirror is gold-plated with BK7 substrate and has a 4 mmdiameter central hole to allow passage of the electron beam,thus allowing the laser pulses to travel collinearly with theelectron beam.
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Figure 5. Laser ray tracing (left, not drawn to scale) and picture of laser dump after line focus (right).
Laser pulses injected into the vapor cell are focused bythe axicon lens to form the plasma. For practical reasons, aconventional spherical lens cannot produce a uniform high-intensity profile over a meter-scale distance but an axicon lens[9] can. The axicon lens also requires lower laser power thanthe conventional spherical lens. Figure 5 illustrates the laserray tracing and the focal region where laser ionization leads toplasma formation. After the line focus, the laser diverges andis dumped onto a glass neutral density filter, which protectsa beryllium vacuum window located further downstream thatis used to isolate experimental vacuum region from the linacvacuum. The pointing stability of the laser focus has beenmeasured to be 37 µrad (RMS) at a location 2.7 m downstreamof the axicon lens.
The length of the plasma column depends on the angleof the axicon lens and the diameter of the laser beam. Usingan axicon lens with a smaller apex angle increases the plasmalength by the ratio of the angles of the lenses. For example,reducing the axicon angle from a 2◦ to 0.5◦ lengthens theplasma from 40 to 160 cm, but requires proportionally morelaser energy to maintain the same peak intensity. The diameterof the laser, which is restricted by the aperture of a mask justin front of the axicon lens, also controls the ionization lengthsince it determines where the focal line ends, as illustrated infigure 5. A smaller circular mask centered on the laser axis atthe same plane as the outer mask likewise determines where thefocal line begins. The mask blocks the central laser rays thatdefine the start of the line focus. The laser energy delivered tothe axicon was about 140 mJ for the first experiment in June2013, and it was about 480 mJ in December 2013 after a laserenergy upgrade.
The E-200 experiments also require spatial overlap of thelaser and the electron beam. A phosphor screen is mountedon the back of the turning mirror in figure 4 to aid in beampositioning. To align the electron beam and the laser along theaxis of the oven, a motorized translation stage moves the ovento the side and substitutes a bypass line with two OTR targetsat locations roughly corresponding to either end of the plasma.A 500 µm-thick titanium disk generates an OTR signal fromthe electron beam and also reflects light from the laser; bothprofiles are measured by a CCD camera at each OTR location.The pointing of the laser is adjusted until the two beams arespatially overlapped to <50 µm at each OTR target. The laserpulses propagate through the axicon lens and form a Besselintensity profile when aligned as expected.
3. Results
A laser-ionized plasma column of 36 cm (FWHM) long witha diameter of about 1.2 mm was formed and used for the E-200 PWFA experiments at FACET. A 1.5◦ axicon lens wasused in combination with a 480 mJ, 42 mm diameter laser pulsecompressed to 250 fs. The diameter of the plasma column wasdetermined by translating the final turning mirror and hencethe laser pulse parallel while observing the accelerated featureof the beam.
The final turning mirror can be extracted to allow the laserpulses to be brought outside of the chamber for equivalent planeimaging. A CCD camera on a rail (figure 4) is positioned at theequivalent locations of the entrance and exit of the oven relativeto the axicon lens. The transverse laser profiles captured bythe camera shown in figure 6 confirm that a Bessel profile ismaintained to the end of the oven. The images in figure 6 weretaken with an attenuated ∼20 mJ laser and the color bar hasbeen scaled to reflect the projected intensity for the full energylaser which is 480 mJ. More importantly, the laser intensity(>1012 W cm−2) is high enough to fully ionize Li vapor in theexperiments with density of 5 × 1016 e cm−3.
When the final turning mirror is inserted, another CCDcamera captures an image of a laser ionization filament(figure 7) through a viewport in the oven bypass line where onlyargon gas is present. The presence of the filament indicatesthat the laser is able to ionize argon. The compressed electronbeam is also strong enough to ionize argon. When the laser andelectron beam are spatially aligned, the two filaments overlapone another.
The latest E-200 experiment observed a clear differencebetween the interaction of the electron beam in a lithiumplasma ionized by the electron beam itself and a plasmagenerated by the laser [4]. More energy gain and moreparticipating charge were observed when the laser was firedinto the lithium vapor ahead of the electron beam. This wasa good indication that the electron beam was going through alaser-ionized lithium plasma. In the two-bunch experiments,the witness bunch was accelerated to higher energies in alaser-ionized plasma due to a longer beam–plasma interactionlength.
The laser intensity required for ionization of lithium,hydrogen and argon is ∼1012 W cm−2, ∼1014 W cm−2, and∼1014 W cm−2, respectively. Since ionizations of argon andlithium were observed, one can expect that hydrogen can alsobe ionized. This shows that PWFA experiments performedwith hydrogen gas are feasible. There is only one electron
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Figure 6. Images of the axicon-focused laser intensity profile: at the entrance of the oven or ∼70 cm from the axicon (left), and the exit ofthe oven or ∼160 cm from the axicon (right). The color bars indicate laser intensities in unit of W cm−2 and show that both profiles containthe required intensity >1012 W cm−2 to singly ionize the lithium vapor.
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Figure 7. Image of the plasma recombination light taken at thebeginning of the axicon focal line shows a laser-ionized filament inthe argon gas. The color bar indicates the light intensity in arbitraryunits.
available to be ionized in hydrogen, and thus experiments canavoid the potential problem of secondary ionization as can bethe case with lithium, cesium or rubidium for example. Theease of bringing hydrogen gas to and from the experimentalchamber and lack of residual or mixed waste are some of theadded advantages that make it an attractive choice for futurePWFA experiments.
4. Conclusion and outlook
A laser system for the first laser-ionized plasma wakefieldacceleration experiments has been successfully commissionedand operated at FACET. The use of an axicon lens wasdemonstrated to focus a laser that ionizes the lithium alkalimetal vapor to form a 36 cm plasma column suitable forPWFA. The E-200 experiments performed at FACET used thistechnique for plasma formation and achieved high-efficiencyacceleration of an electron beam [4]. These experiments usedlithium because of its low ionization potential. However,
a laser intensity of >1014 W cm−2 is sufficient to ionize ahydrogen-filled gas cell as well. Therefore, future PWFAexperiments may choose to use hydrogen as the plasma source.
Most recently, good quality axicon profiles have beencreated over a 1.5 m distance. In the next E-200 experimentalrun, the 40 cm long Li oven will be replaced with one of 1.5 min length. A smaller angle axicon lens will be used for plasmaformation over the increased length. The expectation is thatthese changes to the FACET PWFA experiments will result inenergy gains in excess of 10 GeV.
The pre-ionization for PWFA presented in this paperis an example of just one experimental application of the10 TW laser system. Other applications of high powerlasers with electron beams include Thomson backscatter forbright directional gamma-ray production [10], imaging of theevolving beam-driven plasma structures [11], and Faradayrotation diagnostics of the accelerating beam inside the PWFA[12], as well as applications of synchronized auxiliary laserpulses that have been demonstrated in connection with laser-driven wakes.
In summary, the high power laser opens up numerousexciting possibilities for new experiments at FACET. Oneexample is the Trojan Horse Plasma Wakefield Acceleration[13] where electrons are released in plasma wave with afocused, ∼50 fs, ∼1 mJ, synchronized laser pulse to generatea high brightness witness beam (εn < 10−8 m-rad). Anotherexperiment being tested at FACET is the self-modulation oflong lepton bunches (σz ∼ 500 µm) in a dense laser-ionizedplasma (ne = 0.6–2.3×1017 cm−3) [14]. Experiments withoutplasma including the optical probe [15] and terahertz radiationpump-probe measurements [16] can be performed by utilizingboth the high-field terahertz pulses from the electron beam andthe laser at FACET.
Acknowledgments
The authors would like to thank the SLAC Test FacilitiesDepartment, the Controls Department, the MechanicalFabrication Department, the Sector 0-20 and AcceleratorOperations and Safety Divisions, and the LCLS Laser Science
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& Technology Division for their efforts and support in buildingthe FACET laser system. This work is supported by theUS Department of Energy under contract numbers DE-AC02-76SF00515 and FG02-92-ER40727.
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