Compton scattering of intense laser pulses with ultrarelativistic electron beams has proven to be an attractive source of high-brightness x-rays with keV to MeV energies. This type of x-ray source requires the electron beam brightness to be comparable with that used in x-ray free-electron lasers and laser and plasma based advanced accelerators. We describe the development and commissioning of a 1.6 cell RF photoinjector for use in Compton scattering experiments at LLNL. Injector development issues such as RF cavity design, beam dynamics simulations, emittance diagnostic development, results of sputtered magnesium photo-cathode experiments, and UV laser pulse shaping are discussed. Initial operation of the photoinjector is described

Anderson, S G

Badakov, H

Barty, C J

Frigola, P

Fukasawa, A

Gibson, D J

Hartemann, F V

Messerly, M

OShea, B

Rosenzweig, J B

Shverdin, M

Siders, C W

Tremaine, A M

UNT (University of North Texas) Digital Library

UCRL-PROC-232211COMMISSIONING OF AHIGH-BRIGHTNESS PHOTOINJECTORFOR COMPTON SCATTERING X-RAYSOURCESS. G. Anderson, D. J. Gibson, F. V. Hartemann, M.Messerly, M. Shverdin, C. W. Siders, A. M. Tremaine,C. P. J. Barty, H. Badakov, P. Frigola, A. Fukasawa, B.OShea, J. B. RosenzweigJune 27, 200722nd PAC ConferenceAlbuquerque, NM, United StatesJune 25, 2007 through June 29, 2007Disclaimer   This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.  COMMISSIONING OF A HIGH-BRIGHTNESS PHOTOINJECTORFORCOMPTON SCATTERING X-RAY SOURCES∗S. G. Anderson† , C. P. J. Barty, D. J. Gibson, F. V. Hartemann, M. Messerly, M. Shverdin,C. W. Siders, A. M. Tremaine, LLNL, Livermore, CA 94550, USAH. Badakov, P. Frigola, A. Fukasawa, B. OShea, J. B. RosenzweigUCLA, Los Angeles, CA 90095, USAAbstractCompton scattering of intense laser pulses with ultra-relativistic electron beams has proven to be an attractivesource of high-brightness x-rays with keV to MeV ener-gies. This type of x-ray source requires the electron beambrightness to be comparable with that used in x-ray free-electron lasers and laser and plasma based advanced accel-erators. We describe the development and commissioningof a 1.6 cell RF photoinjector for use in Compton scatter-ing experiments at LLNL. Injector development issues suchas RF cavity design, beam dynamics simulations, emit-tance diagnostic development, results of sputtered magne-sium photo-cathode experiments, and UV laser pulse shap-ing are discussed. Initial operation of the photoinjector isdescribed.INTRODUCTIONBright, monoenergetic beams of gamma-rays (ΔE/E ≤10−3) ranging from hundreds of keV to several MeV willbecome important tools of future experimental investiga-tion. In applications under development at LLNL, this typeof beam will be used to induce and observe nuclear reso-nance fluorescence (NRF) [1]. By tuning the gamma-raybeam energy, NRF can be used to identify particular iso-topes, while the energies of NRF transitions are well suitedto allow the photons to penetrate materials with large arealdensities. Detection systems based on NRF may be appli-cable to areas including nonproliferation, waste identifica-tion, and material characterization.The x-ray source under construction at LLNL, T-REX(Thompson-Radiated Extreme X-rays), will produce 680keV photons by colliding a 112 MeV photo-electron beamwith a 355 nm, joule class laser pulse. Start-to-end simula-tions of this source have shown peak brightnesses of 1022photons/(mm2 ×mrad2 × s× 0.1%bandwidth) [2]. Thebrightness of this source scales inversely with the square ofthe geometric emittance of the electron beam, thus makingit very attractive at higher energies. This scaling also re-veals the fact that optimization of the electron beam bright-ness is critical to the brightness of the photon source. In∗This work was performed under the auspices of the U.S. Departmentof Energy by the University of California, Lawrence Livermore NationalLaboratory under contract No. W-7405-Eng-48, and by the University ofCalifornia, Los Angeles under contract numbers DE-FG-98ER45693 andDE-FG03-92ER40693† anderson131@llnl.govfact, the electron beams parameters required to achievethe photon source brightness given above are comparablewith those of X-ray free-electron lasers and other advancedphoto-beam applications (≈ 1 mm mrad, 1 nC).We describe here the photoinjector designed to gener-ate the electron beam parameters required by T-REX. Ra-dio frequency (RF) design considerations, beam dynamicssimulations, low and high power tests, photo-cathode ex-periments, and emittance diagnostic development work aredetailed. Development of a fiber-based cathode drive laseris described.THE PHOTO-GUNThe T-REX gun, shown in Fig. 1, is a 1.6 cell photo-cathode gun of the BNL/SLAC/UCLA/LLNL design [3].There are however, several key changes from previous de-signs [4]. First, because on-axis laser injection is planned,the 70◦ ports of the half cell were removed, making thehalf cell fully cylindrically symmetric. The full cell wasalso symmetrized to quadrupole moment by replacing thetuning plungers, which have been found to break down athigh field and limit gun performance [5], with race-trackshaped slots identical to the RF coupling slot.Another significant change to the gun design was to in-crease the frequency separation of the 0- and π-modes.This modification was first implemented at SLAC in or-der to minimize the excitation of 0-mode fields which wereFigure 1: The T-REX photo-gun.-25-20-15-10-502840 2845 2850 2855 2860Frequency (MHz)12.3 MHzS11 (dB)Figure 2: Network analyzer measurements of the T-REXgun showing the 0- to π-mode separation increased to 12.3MHz.found to have a detrimental effect on the beam emittanceand energy spread [6]. The mode separation of the T-REXgun was increased to 12.3 MHz. PARMELA simulationswere performed and indicate that the effect of the 0-modeis negligible at the larger mode separation, in agreementwith the results of Ref. [6].Cavity resonance and bead pull measurements were per-formed before and after brazing to ensure that π-modefield balance and frequency could be properly adjusted us-ing only cathode plate deformation and temperature tuning.These measurements agreed well with 3-D HFSS simula-tions. The reflected power measurements of the gun in-stalled at LLNL are shown in Fig. 2. Bead pull data in-dicates better than 1% field balance at the measured 12.3MHz mode separation.The photo-cathode, pictured in Fig. 3, consists of a cop-per disk which forms the half cell back plane, with a 2μmsputter-coated layer of Mg deposited in a 1cm diameter cir-cle in the center. This cathode preparation technique buildson previous efforts [7], but with the key difference of a150A˚‘adhesion-layer’ of Cr used between the Cu and Mg.The T-REX gun with sputtered Mg cathode has been RFconditioned at LLNL up to 8 MW of peak power, corre-sponding to approximately 107 MV/m peak acceleratingfield. Further conditioning is possible but will not be doneuntil the performance of the gun and cathode can be fullyevaluated. A sputteredMg cathode was installed in an olderversion of the 1.6 cell gun and conditioned to 80 MV/mpeak field (limited by tuner break-downs). The quantumefficiency (QE) of the cathode was measured using a fre-quency tripled Ti:Sapphire laser, injected 40 RF degreesfrom the zero crossing. The UV laser energy was thenscanned and beam charge measured. The results, plottedin Fig. 4, show a QE of 1× 10−4 at low charge. At chargesFigure 3: Image of the sputter-coated Mg photo-cathode.05001000150020000 25 50 75 100 125 150UV Energy (µJ)QE = 1.0e-4Charge (pC)Figure 4: Charge produced using a sputtered Mg cathodeplotted as a function of UV laser energy. The peak acceler-ating field for this measurement was 80 MV/m (limited bytuner break-down in an older gun design) and the laser wasinjected at 40◦.above 1 nC, the QE falls off due to the effect of longitudi-nal space-charge fields. This result is consistent with previ-ous data reported for friction-welded Mg cathodes withoutlaser cleaning [8].The high QE of Mg enables the use of a fiber-basedphoto-cathode drive laser system (PDL). The T-REX PDL[9] will generate up to 500 μJ of 1047 nm, 1 ps durationlight. The laser is then frequency quadrupled, sent througha pulse stacking interferometer to generate an 8 ps flat-toptemporal profile, and relay imaged through an iris to thephoto-cathode. In this way the ‘beer-can’ laser pulse shaperequired for emittances approaching 1 mm mrad at 1 nC ofcharge will be achieved.00.511.522.533.540 100 200 300 400Z(cm)GunAccelerator SectionEmittance (mm mrad)x RMS (mm)Figure 5: PARMELA simulation of the T-REX photoinjec-tor. One nC is accelerated to 30MeV using the gun and firstaccelerator section. The final normalized, rms emittance isbelow 1 mm mrad.THE PHOTOINJECTOR BEAMLINEThe T-REX accelerator will generate >100 MeV elec-trons using 5 SLAC-type 2.5 meter traveling-wave sec-tions. The transport line from the photo-gun to the firstaccelerator section has been designed both to minimize theemittance at the end of the machine and to fully diagnosethe beam at low energy.A modified version of the LCLS working point [10]is employed, which uses an 8 ps (FWHM) stacked-pulselaser profile as is anticipated using the UV interferome-ter. The radial profile modeled is a Gaussian cut-off at1σ. The PARMELA simulation shown in Fig 5 tracks thebeam size and normalized, rms emittance through the gun,150cm drift region and the first accelerator section. Asolenoid placed immediately downstream of the gun causesthe beam to execute the typical emittance compensation os-cillation [11] in both beam size and emittance, with emit-tance minima occurring at both the entrance and exit of thelinac section.Note that the beam is slightly converging at the entranceof the accelerator section instead of the typical design ofa waist. This mismatch causes the beam size to oscillateabout the invariant envelope in the section and is done tocompensate for the fact that no solenoid focusing at thesections is employed. This simulation accelerates 1 nC ofcharge to 30 MeV, and produces a final normalized emit-tance of 0.7 mm mrad. No significant change in the emit-tance occurs with the addition of the remaining sectionsneeded to accelerate to the T-REX design energy of 112MeV.The 150cm drift from photo-cathode to linac section re-quired for proper emittance compensation is utilized to di-agnose the electron beam produced by the gun. Chargeis measured single-shot and non-destructively with an in-tegrating current transformer (ICT). In addition, diagnos-tics exist to measure beam energy and profile, as well asFigure 6: Comparator image of laser drilled ‘pepper-pot’beam mask. An array of 20μm holes in a 0.5 mm Tungstenplate are spaced by 0.75 mm.RF and laser parameters. Most importantly, the trans-verse emittance (and trace space) will be measured usinga ‘pepper-pot’ beam mask (2-D analog to the slit mask)[12]. By observing the down-stream profile of beamletspassing through the mask the beam trace space, and there-fore emittance may be reconstructed on a single-shot basis.The beam mask shown in Fig. 6 consists of an array of20μm holes laser drilled into a 0.5mm thick tungsten plate.The geometry of the emittance diagnostic was chosen us-ing the analysis of Ref. [12] to best measure normalizedemittances in the range of 0.5 to 5 mm mrad.REFERENCES[1] J. Pruet et al., J. Appl. Phys. 99, 123102 (2006).[2] F. V. Hartemann et al., Phys. Rev. ST Accel. Beams 8,100702 (2005).[3] D. T. Palmer, Ph. D. thesis, Stanford University (1998).[4] A. Fukasawa et al., these proceedings.[5] A.M. Cook et al., The Physics and Applications of HighBrightness Electron Beams, Erice, Sicily (2005).[6] C. Limborg et al., SLAC Technical Note LCLS-TN-05-3(2005).[7] T. Srinivasan-Rao et al., Rev. Sci. Instrum. 69, 2292 (1998).[8] X.J. Wang et al., Proc. of LINAC 2002, pp. 142;H. Iijima et al., Proc. of LINAC 2002, pp. 166;J.F. Schmerge et al., Proc. of the 2004 FEL Conference, pp.205.[9] D.J. Gibson et al., these proceedings.[10] M. Ferrario et al., Proc. of the 2nd ICFA Adv. Acc. Work-shop on “The Physics of High Brightness Beams” (1999).[11] L. Serafini and J.B. Rosenzweig, Phys. Rev. E 55 7565(1997).[12] S.G. Anderson et al., Phys. Rev. ST Accel. Beams 5, 014201(2002).

COMMISSIONING OF A HIGH-BRIGHTNESS PHOTOINJECTOR FOR COMPTON SCATTERING X-RAY SOURCES

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Commissioning of a high-brightness photoinjector for compton scattering x-ray sources

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COMMISSIONING OF A HIGH-BRIGHTNESS PHOTOINJECTOR FOR COMPTON SCATTERING X-RAY SOURCES

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