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High resolution resonant recombinationmeasurements using evaporative coolingtechniqueTo cite this article C Beilmann et al 2010 JINST 5 C09002

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2010 JINST 5 C09002

PUBLISHED BY IOP PUBLISHING FOR SISSA

RECEIVED July 1 2010ACCEPTED July 21 2010

PUBLISHED September 9 2010

INTERNATIONAL SYMPOSIUM ON ELECTRON BEAM ION SOURCES AND TRAPSAPRIL 7thndash10th 2010STOCKHOLM SWEDEN

High resolution resonant recombinationmeasurements using evaporative cooling technique

C Beilmann 1 JR Crespo L opez-Urrutia PH Mokler and J Ullrich

Max-Planck-Institut fur KernphysikSaupfercheckweg 1 69117 Heidelberg Germany

E-mail christianbeilmannmpi-hdmpgde

ABSTRACT We report on a method significantly improving the energy resolution of dielectronicrecombination (DR) measurements in electron beam ion traps(EBITs) The line width of DR reso-nances can be reduced to values distinctly smaller than the corresponding space charge width of theuncompensated electron beam The experimental technique based on forced evaporative cooling ispresented together with test measurements demonstrating its high efficiency The principle for res-olution improvement is elucidated and the limiting factorsare discussed This method opens accessto high resolution DR measurements at high ion-electron collision energies required for innermostshell DR in highly charged ions (HCI)

KEYWORDS Low-energy ion storage Plasma diagnostics - interferometry spectroscopy andimaging Ion sources (positive ions negative ions electron cyclotron resonance (ECR) electronbeam (EBIS))

1Corresponding author

ccopy 2010 IOP Publishing Ltd and SISSA doi1010881748-0221509C09002

2010 JINST 5 C09002

Contents

1 Introduction 1

2 Resolution of DR measurements 3

3 Electron energy resolution improvement 331 Standard methods for resolution improvement 432 Resolution improvement by evaporative cooling 5

4 Experimental results 6

5 Conclusion and outlook 6

1 Introduction

A wealth of crucial physical investigations was enforced bythe study of dielectronic recombination(DR) In this process a free electron interacts with an atomic one bound by the positive potential ofa highly charged ion (HCI) whereby its kinetic energy is directly transfered to the bound electron ofthe ion This dielectronic capture is a resonant process andleads usually to a doubly excited stateThe so-formed doubly excited state can deexcite radiatively to the ground state thus completing therecombination The strong increase of the electron capturecross section for resonant recombinationcompared to the continuous behavior of radiative recombination makes DR measurements to asuitable tool for atomic structure investigations of innershells in particular for studying quantumeffects interesting for fundamental atomic physics [1 2] for nuclear charge size investigations [3]and makes it also to a technical tool for control and analysisof nuclear fusion plasmas [4 5] toname but a few as applications of DR measurements In many cases a precise determination of theresonance energies is essential for these experiments demanding for the highest energy resolutionin DR measurements

In order to measure DR HCIs have to be provided and brought tocollisions with electronsunder extremely well defined energy conditions Experimentally this is realized in two differentways For the low collision energy range (0 to almost 2000 eV)fast ions are usually provided instorage rings (SR) and merged colinearly with electron beams of a dedicated electron target or anelectron cooler There DR is usually detected by monitoringthe down-charged ions as function ofthe relative collision energy (eg [6]) For the high collision energy range ie in the region beyondsim1 keV and up to the highest energies the best results on DR have been achieved using electronbeam ion traps (EBITs) [7] investigating for trapped ions (almost at rest) electron-ion collisions bymonitoring the stabilizing photon emission as a function ofthe electron beam energy

While in SR measurements due to the expansion of the energy scale caused by the transfor-mation from the comoving fast ion frame to the lab frame the resolution reaches at low resonance

ndash 1 ndash

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93 94 95 96

Electron beam energy (keV)

Figure 1 KLL DR resonances in the energy range for B- to N-like Kr ions the focus in this work is theinvestigation of C-like ions (marked red region) At the topa two-dimensional intensity plot is shown thespectrum below is a projection on the electron beam energy axis

energies the sub-eV region the resolution in EBIT measurements ie for ions at rest shows athigh resonance energies values of few 10 eV This fact complicates the investigation of weak ef-fects in the high energy region where EBITs preferrably are used In this contribution we discussthe resolution of DR resonances measured in EBITs and present a method to improve the resolutionconsiderably for this kind of measurements

In an EBIT HCIs are produced by electron impact ionization by a dense electron beam com-pressed by the magnetic field of a superconducting magnet The electron beam confines the ions inradial direction by its strong negative space charge The axial confinement is ensured by a trappingpotential applied to a set of in principle three drift tubes around the reaction volume

In DR measurements the fast electrons serve both the needs for production of the ions andas collision partner for the DR process Variing the energy of the electron beam the electrons canrecombine around the resonance with the ions Via energy selective measurement of the photonsradiated during stabilization of the excited ions the recombination is detected an increase ofphotons having the transition energy of the excited state isthe signature for a DR resonance Thiscan be clearly seen by plotting the photon intensity in a two-dimensional representation versuselectron beam energy and photon energy around the DR resonances [8] cf figure1 In the figurethe region around C-like DR resonances in Kr30+ for then = 2rarr n = 1 photon transitions at around13 keV is shown together with the neighbouring B- and N-likesystems

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2 Resolution of DR measurements

The energy resolution of DR resonances in EBIT measurementsis dominated by the space chargeof the electron beam represented by the space charge potential [9]

Vsp(r) =Ie

4πε0vemiddot

)2+ ln

)2minus1 for r le re

for r ge re

Consequently the space charge potential of an electron beam with a currentIe and a radiusre

depends on the distancer to the axis of the beam inside a drift tube with radiusrdt Accordingto the number and charge state of the stored ions this potential is partially compensated leadingto an effective space charge potentialVeff(r) = Vsp(r) middot f The compensation factorf is given byf = sumqnqq middot nminus1

e wherenq represents the number of ions with a positive charge stateq andne

stands for the number of negative electrons [10] This compensation is difficult to estimate due tothe required knowledge of the exact charge state distribution of the ions and their quantity in theEBIT both depending on operational parameters (see also [11]) Moreover also ions of the residualgas contribute to the compensation additionally complicating the modelling for the space chargecompensation

The energy of the electrons is defined by the accelerating potentials at the drift tubes reducedby the effective space charge Accordingly the electrons have different energies concerning differ-ent distances to the beam axis despite seeing the same applied accelerating potentials Since onlythese potentials can be controlled and measured the actualenergy distribution of the electronsdominates the measured resolution at the resonance energies

Beyond current and electron energy the space charge dependson device-specific parameterslike the radius of the drift tubes and the radius of the electron beam that is again governed bythe compressing magnetic field and specifications of the electron gun [12] There are differentresolutions achieved in various EBITs built for diverse scientific needs In order to discuss theachieved resolution device independently we define a resolution parameterR as ratio between theFWHM (full width at half maximum) of the measured resonance∆Eexp and the calculated energydifference inside the electron beam caused by the uncompensated space charge potential difference∆VSP between the beam axis and the edge of the electron beam

R=∆Eexp

emiddot∆VSP(22)

For the radius of the electron beam we use for convenience here the Herrmann radius [12]

3 Electron energy resolution improvement

An optimal absolute resolution can be achieved by limiting the movement of the ions to a minimalaxial volume so that they are only hit by electrons nearest to the electron beam axis The naturalvolume where ions can be confined to is that of the electron beam itself because the ions areproduced in the whole beam volume To minimize the kinetic energy and thus the movement of theions the temperature development of the ions has to be considered in detail According to [9] theions are heated by impact with the electrons of the beam and bycollisions with other ions wherebydue to the loss of energetic ions in axial and radial direction the ions are cooled

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this work

Measurement

Berlin Heidelberg

respective space charge

of the electron beam

Figure 2 Comparison of the resolution parameterR= ∆Eexpemiddot∆Vsp the ratio of the experimental resolution(FWHM) for C-like KLL DR resonances in Kr to the calculated resolution due to the space charge widthof the uncompensated electron beam for several measurements in different EBITs [13ndash18] The differentlaboratories are indicated at the top While measurements using the standard method (diamonds) obtain aratio between 2 and 9 the measurement using forced evaporative cooling reaches a ratio of 046 (red dot)

31 Standard methods for resolution improvement

In previous measurements of DR resonances in EBITs both space charge and heating of ionswere minimized by reducing the electron beam current due to its linear dependence on the electrondensity [9] In particular the space charge potential itself is reduced and consequently also theabsolute energy width of the electron beam depending linearly on the beam current (eq (21)) Uti-lizing this space charge minimizing strategy resolution parametersR between 2 and 9 have beenreached for measurements of KLL DR resonances in krypton used for comparison here cf figure2mdash for further illustration of the method see also below figure4a This implies that the ion cloud oc-cupies obviously a bigger volume than the electron beam itself The high R values beyond 1 implythe effective electron beam radius may deviate from the Herrmann radius comprising nominallyonly 80 of the electron beam

The maximal attained space charge reduction by minimizing the electron beam current is lim-ited by different factors A reduction of the electron beam means a considerably less efficient ion-ization of the target gas by electron impact The highest charge states required for measurements ofKLL DR resonances are hardly reachable at low electron beam currents hence the current cannotbe reduced gratuitously Moreover a reduced electron intensity at low beam currents means a lowrecombination rate resulting in weak counting rates on top to the reduced ion density The reduc-tion of the radial space charge potential by lowering the electron current results beside a narrowerenergy width in a weaker confinement of the already sparse ions Hence this leads to a weakerspace charge compensation

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Offset voltage on central drift tibe (V)

940 945 950

Figure 3 Improvement of KLL DR resolution (∆Eexp FWHM) in C-like Kr ions by evaporative coolingraising the voltage on the central drift tube the space charge generated rdquonaturalrdquo axial trapping potential iscompensated forcing the escape of the hottest ions in axial direction The diagram shows the reduction ofthe resolution (FWHM) of the first C-like DR resonance([1s2s22p2

12(2p232)2]52) as function of the offset

voltage at the central drift tube ie the positive voltageat the central drift tube with respect to the surroundingones The 3-dimensional illustrations on the top show theseresonances (Eγ asymp 13 keV Ee asymp 945 keV) at30V and 140V offset voltage the intensity for the recombination radiation is displayed as function of photonenergy and electron beam energy

32 Resolution improvement by evaporative cooling

The alternative method presented in this paper is the forcedcooling of the ions The present coolingtechnique opposes the space charge minimizing method applied previously Instead of a reductionof the space charge cooling of the ions by forced evaporation [19] is promoted Here the axialconfinement of the particles is reduced to cool them by the loss of the most energetic ones In [20]a method of evaporative cooling in an EBIT is discussed for the goal of producing the highestcharge states of heavy ions using a cooling gas In contrast to this in the present experimentsthe trap potential was optimized to maximal cooling in such away that the axial confinement wasreduced to a point where just the coldest ions are hold at the bottom of the trap with the objective ofresolution improvement Simultaneously a high electron beam current ofI = 200 mA was chosen toensure both a high ionization and recombination efficiency of the strongly confined ions in radialdirection With this it is additionally ensured that enoughatoms of the target gas as well as theresidual gas are ionized to reach a maximal compensation of the space charge further minimizingthe energy distribution of the ions

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The axial trap in an EBIT controlling the loss of the hottest ions is generated by positivepotentials at the surrounding drift tubes relative to the central one containing the reaction volumeBy applying equal potentials at all drift tubes there is still a trapping potential due to the dependenceof the space charge potential on the radius of the drift tubes(eq (21)) because in common EBITdesigns the surrounding drift tubes have smaller radii thanthe central one In our case the radiusof the central drift tube isrc = 5 mm where the surrounding ones have a radius ofrs = 15 mm Tocompensate this potential the voltage on the central drifttube was raised to a point where nearly norelaxation photons from DR were detected indicating a totally emptied trap Close to this thesholdthe weak trap is ideal for a strong radial confinement of the coldest ions only

4 Experimental results

To demonstrate the resolution improvement we have examined KLL DR resonances of kryptonin the Heidelberg-EBIT [21] The DR resonances in the energy region for N-like to O-likeDRwhere measured with different offset voltages on the central drift tube ie the positive voltage onthe central drift tube with respect to the surrounding onesThe measurement itself was performedby varying the electron beam energy in sawtooth ramps over the range of the expected resonanceenergies with a ramp velocity of 2 Vs to ensure a quasi-static charge state distribution The elec-tron energy was calibrated to the theoretical energies of C-like and N-like DR resonances takenfrom [22] The photons were detected with a high purity germanium x-ray detector and their num-ber in the energy region of then = 2 rarr n = 1 transitions (around 13 keV) was projected on theelectron energy axis cf figure1 Raising the voltage on the central drift tube the energy resolutionof the DR resonances improves significantly Figure3 shows the evolution of the resolution in de-pendence on the offset voltage of the central drift tube In a3-dimensional representation photoncounts for the C-like K-LL DR resonances are displayed as a function of electron beam energy andphoton energy Resonances that are not resolved by the classical measuring technique are clearlyseparated now by applying this method of forced evaporativecooling

The resolution progresses to values markedly smaller than the energy distribution of the un-compensated electron beam in our case down to 15 eV for an electron beam space charge potentialof 32 eV (cf figure3) This leads to an excellent resolution parameterR down to 046 indicating aminimal radial movement area for the ions and simultaneously to a high space charge compensa-tion of more than 50 The cooling is so effective that all theions are confined inside the electronbeam and thus the method further uses the high ion productionrate for space charge compensa-tion minimizing the electron energy spread Although good absolute energy resolutions have beenachieved in some EBITs [15] due to their individual device specifications non of thesemeasure-ments had resolved the compared resonances to values smaller than those corresponding to thespecific electron beam space charge widths (cf figure2)

5 Conclusion and outlook

The method described above mdash utilyzing forced evaporative cooling in an EBIT mdash improves forDR measurements the relative resolution represented by theresolution parameterR = ∆Eexpemiddot∆Vsp (eq (22)) by a factor of 4 to 20 (cf figure2) The advantage of the forced cooling technique

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2010 JINST 5 C09002

Axial potential Radial potential

4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

Figure 4 Illustration of the two methods for resolution improvement a) standard technique applying e-beam dilution and b) forced axial evaporative cooling technique with dense electron impact Axial andradial potentials and their changes are schematically displayed (left and right side respectively) For case a)the space charge potential is reduced by e-current reduction without touching the axial potentials (see redrarr blue curves) For case b) high e-current and space charge arenot changed however the axial potentialwell is reduced allowing forced evaporative cooling and thus concentrating the ions of interest radially to areduced volume (see redrarr blue curves the vertical two-sided arrows display the change in resolution)

introduced for resolution improvement over the previouslyused standard method of space chargeminimizing by electron beam dilution is summarized in figure4 (b and a respectively) For thestandard method (figure4a) only strong resonances can be measured due to the lack of intensity andthis only at a sparce charge limited resolution In contrastwith the forced axial evaporative coolingtechnique (figure4b) using dense electron impact faint structures are accessible at high resolutionA first result achieved in the Heidelberg-EBIT by means of this method is the evidence of KL-LLLtrielectronic recombination in highly charged krypton ions [22] This method can be utilized inother EBITs too The forced evaporative cooling techniqueapplied offers new opportunities instudying DR or other processes with high resolution and thisfor high resonance energies too

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2010 JINST 5 C09002

References

[1] Z Harman et alBenchmarking high-field few-electron correlation and QED contributions in Hg75+

to Hg78+ ions II Theory Phys RevA 73 (2006) 052711

[2] AJ Gonzalez Martinez et alState-Selective Quantum Interference Observed in the Recombinationof Highly Charged Hg75+78+ Mercury Ions in an Electron Beam Ion TrapPhys Rev Lett94 (2005) 203201

[3] C Brandau et alIsotope Shift in the Dielectronic Recombination of Three-ElectronANd57+Phys Rev Lett100 (2008) 073201

[4] M Bitter et alSpectra of heliumlike krypton from Tokamak Fusion Test Reactor plasmasPhys Rev Lett71 (1993) 1007

[5] K Widmann et alStudies of He-like krypton for use in determining electron and ion temperatures invery-high-temperature plasmas Rev Sci Instrum66 (1995) 761

[6] D Nikolic et alDielectronic recombination resonances in Na8+ Phys RevA 70 (2004) 062723

[7] MA Levine RE Marrs JR Henderson DA Knapp and MB SchneiderThe Electron Beam IonTrap a New Instrument for Atomic Physics Measurements Phys ScriptaT 22 (1988) 157

[8] DA Knapp et alDielectronic recombination of heliumlike ions Phys RevA 47 (1993) 2039

[9] BM Penetrante et alEvolution of ion-charge-state distributions in an electron-beam ion trapPhys RevA 43 (1991) 4861

[10] K WidmannHigh-Resolution Spectroscopic Diagnostics of very High-Temperature Plasmas in theHard X-ray Regime PhD Thesis US Department of Energy Lawrence Livermore NationalLaboratory (1999)

[11] XLu et alNumerical simulation of the charge balance and temperatureevolution in an electronbeam ion trap Phys Rev ST12 (2009) 014401

[12] G HerrmannOptical Theory of Thermal Velocity Effects in Cylindrical Electron BeamsJ Appl Phys29 (1958) 127

[13] T Fuchs et alChannel-specific dielectronic recombination of highly charged kryptonPhys RevA 58 (1998) 4518

[14] R Radtke et alMeasurement of the radiative cooling rates for high-ionization species of kryptonusing an electron beam ion trap Phys RevE 61 (2000) 1966

[15] M Schmidt et alReport on the current developments and experiments of the Dresden EBIT SystemJ Phys Conf Ser72 (2007) 012020

[16] Z Hu et alMeasurement of the KLL Dielectronic Recombination Resonances in He-like to C-like KrIons Chin Phys Lett26 (2009) 033401

[17] JR Crespo Lopez-Urrutia et alOptimization of the charge state distribution of the ion beamextracted from an EBIT by dielectronic recombination Rev Sci Instrum75 (2004) 1560

[18] AJ Gonzalez MartınezQuantum interference in the dielectronic recombination ofheavy highlycharged ions PhD Thesis University of Heidelberg (2005)

[19] CE KlotsEvaporative cooling J Chem Phys83 (1985) 5854

[20] BM Penetrante et alEvaporative cooling of highly charged dysprosium ions in anenhancedelectron-beam ion trap Phys RevA 43 (1991) 4873

[21] JR Crespo Lopez-Urrutia et alThe Freiburg Electron Beam Ion TrapSource Project FreEBIT

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2010 JINST 5 C09002

Phys ScriptaT 80 (1999) 502

[22] C Beilmann et alIntershell trielectronic recombination with K-shell excitation in Kr30+Phys RevA 80 (2009) 050702

ndash 9 ndash

Introduction

Resolution of DR measurements

Electron energy resolution improvement

Standard methods for resolution improvement

Resolution improvement by evaporative cooling

Experimental results

Conclusion and outlook

2010 JINST 5 C09002

PUBLISHED BY IOP PUBLISHING FOR SISSA

RECEIVED July 1 2010ACCEPTED July 21 2010

PUBLISHED September 9 2010

INTERNATIONAL SYMPOSIUM ON ELECTRON BEAM ION SOURCES AND TRAPSAPRIL 7thndash10th 2010STOCKHOLM SWEDEN

High resolution resonant recombinationmeasurements using evaporative cooling technique

C Beilmann 1 JR Crespo L opez-Urrutia PH Mokler and J Ullrich

Max-Planck-Institut fur KernphysikSaupfercheckweg 1 69117 Heidelberg Germany

E-mail christianbeilmannmpi-hdmpgde

ABSTRACT We report on a method significantly improving the energy resolution of dielectronicrecombination (DR) measurements in electron beam ion traps(EBITs) The line width of DR reso-nances can be reduced to values distinctly smaller than the corresponding space charge width of theuncompensated electron beam The experimental technique based on forced evaporative cooling ispresented together with test measurements demonstrating its high efficiency The principle for res-olution improvement is elucidated and the limiting factorsare discussed This method opens accessto high resolution DR measurements at high ion-electron collision energies required for innermostshell DR in highly charged ions (HCI)

KEYWORDS Low-energy ion storage Plasma diagnostics - interferometry spectroscopy andimaging Ion sources (positive ions negative ions electron cyclotron resonance (ECR) electronbeam (EBIS))

1Corresponding author

ccopy 2010 IOP Publishing Ltd and SISSA doi1010881748-0221509C09002

2010 JINST 5 C09002

Contents

1 Introduction 1

1 Introduction

ndash 1 ndash

2010 JINST 5 C09002

93 94 95 96

ndash 2 ndash

2010 JINST 5 C09002

Vsp(r) =Ie

4πε0vemiddot

)2+ ln

for r ge re

R=∆Eexp

emiddot∆VSP(22)

ndash 3 ndash

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this work

Measurement

Berlin Heidelberg

ndash 4 ndash

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30 50 70 90 110 130 15010

940 945 950

ndash 5 ndash

2010 JINST 5 C09002

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4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

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2010 JINST 5 C09002

References

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Introduction

2010 JINST 5 C09002

Contents

1 Introduction 1

1 Introduction

ndash 1 ndash

2010 JINST 5 C09002

93 94 95 96

ndash 2 ndash

2010 JINST 5 C09002

Vsp(r) =Ie

4πε0vemiddot

)2+ ln

for r ge re

R=∆Eexp

emiddot∆VSP(22)

ndash 3 ndash

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this work

Measurement

Berlin Heidelberg

ndash 4 ndash

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30 50 70 90 110 130 15010

940 945 950

ndash 5 ndash

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4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

ndash 7 ndash

2010 JINST 5 C09002

References

ndash 8 ndash

2010 JINST 5 C09002

ndash 9 ndash

Introduction

2010 JINST 5 C09002

93 94 95 96

ndash 2 ndash

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Vsp(r) =Ie

4πε0vemiddot

)2+ ln

for r ge re

R=∆Eexp

emiddot∆VSP(22)

ndash 3 ndash

2010 JINST 5 C09002

this work

Measurement

Berlin Heidelberg

ndash 4 ndash

2010 JINST 5 C09002

30 50 70 90 110 130 15010

940 945 950

ndash 5 ndash

2010 JINST 5 C09002

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4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

ndash 7 ndash

2010 JINST 5 C09002

References

ndash 8 ndash

2010 JINST 5 C09002

ndash 9 ndash

Introduction

2010 JINST 5 C09002

Vsp(r) =Ie

4πε0vemiddot

)2+ ln

for r ge re

R=∆Eexp

emiddot∆VSP(22)

ndash 3 ndash

2010 JINST 5 C09002

this work

Measurement

Berlin Heidelberg

ndash 4 ndash

2010 JINST 5 C09002

30 50 70 90 110 130 15010

940 945 950

ndash 5 ndash

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4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

ndash 7 ndash

2010 JINST 5 C09002

References

ndash 8 ndash

2010 JINST 5 C09002

ndash 9 ndash

Introduction

2010 JINST 5 C09002

this work

Measurement

Berlin Heidelberg

ndash 4 ndash

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30 50 70 90 110 130 15010

940 945 950

ndash 5 ndash

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ndash 6 ndash

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4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

ndash 7 ndash

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References

ndash 8 ndash

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Introduction

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30 50 70 90 110 130 15010

940 945 950

ndash 5 ndash

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2010 JINST 5 C09002

4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

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References

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Introduction

2010 JINST 5 C09002

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4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

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References

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Introduction

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4 cm axial

trap length

resolution with

e-beam dillution

50 microm

e-beam diameter

resolution with

evaporative cooling

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References

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High resolution resonant recombination measurements - IOPscience