The Cornell University Photoinjector

Transcription

1 The Cornell University Photoinjector L. Cultrera On behalf of ERL team

2 Outline The Cornell ERL R&D program The photoinjector Photocathode lab SRF development Achieved Milestones Beam brightness and High average current operation New photocathode materials SRF HTC Cryomodule The future New DC gun Photocathode engineering ERL test loop

3 Layout of Cornell ERL CESR ERL main linac: 5 GeV, 100 ma with energy recovery ERL injector Undulators and X-ray beam lines Beam Stop

4 The photo-injector 20m Parameter Metric Status Notes Average Current 100 ma 65 ma at 4 MeV (1300 MHz) Bunch Charge 77 pc Pulsed mode (50 MHz) Energy 5 to 15 MeV 14 MeV max (due to cryo limits) Laser Power > 20 W > 60 W at 520 nm (1300 MHz) Laser Shaping beer can dist. Adequate for now Gun Voltage kv Currently operating at 350 kv Emittance <0.5 mm (norm, rms) Ultimate ERL goal 0.3 mm, with merger Operational Lifetime > 1 day Recent improvements with new cathodes

5 Ideal Loop 200-CPU computer cluster for beam dynamics simulations BPM non linear response Numerical simulation for optimizing machine setting 475kV gun Emittance Measurement System Fine tuning of injector physical modeling and measurement tool Measurement of beam properties Time-domain diagnostics: 0.1 ps RF quad due to input couplers Comparison of experimental data and numerical simulation predictions Actual measurements GPT simulation

6 Photocathodes During last years we have been designing and realizing different separated UHV chamber: Alkali antimonides GaAs Vacuum Suitcase Auger- LEED Electron Energy Analyzer One of the key elements to work with high quantum efficiency photocathode is vacuum. Vacuum level in our UHV system is in few Torr photoinjector Extremely time consuming Vacuum not good enough for GaAs Build a permanent UHV connection!!

7 Molecular beam epitaxy for low emittance materials MBE reactor in Phillips Hall control band-bending by profiled doping MBE: ultimate tool for photocathodes Atomically flat GaAs photocathodes with customized doping Routinely used in photoinjector for smallest emittance Now engineering new types of MBE photocathode structures

8 Photoemission modeling Monte-Carlo simulation tool for III-V family photocathodes Fully developed for non-layered reflective cathodes, now working on layered & transmission mode structures

9 Drive Laser After recent improvements, we increased the average power from < 15 W at 520 nm to over 60 Watts! Added a second pre-amp, compressed the pulse after the amplifier to reduce nonlinearities instead of before it, and starting using commercial, high-power fiber terminators. Commercial fiber terminator Now, we have more headroom for dealing with cathode lifetime, and shaping and transport losses

10 Halo & Vacuum Laser Mirror Image on the cathode using normal dielectric mirror Image on the cathode using coated metal mirror Vac. mirror: ~5% in halo Normal mirror: ~0.1% in halo Our final laser mirror scattered ~50x more light compared to dielectric mirrors (which we cannot use). A new mirror with 2 nm-rms surface roughness was installed to fix the problem Following work by a group at DESY

11 Main Linac Cavity Development and high Q 0 Optimized cavity shape robust up to 0.25 mm shape imperfections! Cell shape optimization: ~20 free parameters Full HOM characterization (1000 s of eigenmodes) Verification of robustness of cavity design (384 for the ERL) Cleaning procedure is kept as simple as possible Two prototypes fabricated and tested successfully Tested up to full power specification of 5 kw CW

12 Milestones

13 Beam emittance at the merger 20 pc/bunch 80 pc/bunch vertical phase space vertical phase space Normalized rms emittance (horizontal/vertical) 90% beam, E ~ 8 MeV, 2-3 ps rms 0.22/0.15 mm-mrad Normalized rms core emittance core fraction (%) 0.14/ % 0.49/0.29 mm-mrad 0.24/ % 100x the brightness at 5 GeV of the best storage ring (1nm-rad hor. emittance 100 ma)! Similar to the best NCRF guns emittance but with > 10 6 repetition rate (duty factor = 1) Courtesy of I. Bazarov

14 Comparison between simulation and measurements A quite good agreement has been achieved between the measurements and numerical beam dynamics simulation t rms = 2.1, 2.2 ps (data, pc/bunch t rms = 3.0, 3.1 ps (data, 80 pc/bunch Courtesy of C. Guilford

15 CsK2Sb Low average current QE map on 03 Feb 2012 ~10% QE map on 03 Apr 2012 ~8% QE map on 02 Oct 2012 ~5% 1/e lifetime ~13 months

16 Lifetime Study with Cs 3 Sb To obtain reasonable lifetime, we developed alkali-type cathodes with the active area off-center, to avoid ion damage. Using a Cs 3 Sb cathode, we ran at 33 ma (4 MeV) for nearly 4 hours no QE degradation observed! This results already exceeds the requirements for the ERL high-coherence mode (25 ma, 19 pc) Courtesy of B. Dunham

17 Highest current with CsK 2 Sb Nov 2, 2012 Using a CsK 2 Sb cathode (offset active area), ran at 60 ma for 25 minutes (30 hr 1/e lifetime, this was the longest uninterrupted time span). Went as high as 65 ma for a short time. Quantum efficiency map of the cathode after growth (left) and after (right) running 60 ma. Delivered over 2000 C from a single 2.5 mm spot. Exceeded the 1993 Boeing results by 2X! Courtesy of B. Dunham

18 Alkali antimonides growth and characterization CsK 2 Sb Cs 3 Sb A suitable recipe has been tested for growing Na 2 KSb photocathodes 532nm is above 5% Full characterization on DC gun (thermal emittance, response time, lifetime) is foreseen

19 Thermal emittance of MBE grown GaAs Typical MTEs of CsK 2 Sb and Cs nm Achieved MTEs lower by a factor 2 w.r.t. bulk alkali antimonide photocathodes

20 Monte Carlo photoemission simulation Simulations explain existing experimental data for bulk GaAs taken by our group without free fit parameters! Will serve as a indispensable tool in guiding us towards novel photocathode structures Spectral response vs affinity Mean Transverse Energy Fully developed for non-layered reflective cathodes now working on layered & transmission mode structures

21 Full System Test of a 1-Cavity Main Linac Unit in a Test Cryomodule 80K shield HGRP First full main linac system test 1 st test: cavity and tuner only (completed) 2 nd test: added high power RF input coupler (completed) Gate valve HOM load cavity HOM load 3 rd test: add HOM beamline loads (currently under assembly) Test cryomodule installed at Wilson Lab Cavity exceeds ERL gradient and Q 0 specifications: Q 0 = 4 to at 1.6K in a cryomodule! World record Q 0 at 1.8K, 16 MV/m in cryomodule!

22 The future

23 Brightness limit formulated Given a laser, photocathode material, and accelerating gradient max brightness is set Each electron bunch assumes a pan-cake shape near the photocathode for short ( 10ps) laser pulses Maximum charge density determined by the electric field: v dq/da = 0 E cath Angular spread set by mean transverse energy (MTE) of photoelectrons Dp ~ (m MTE) 1/2 B n f max = 0mc 2 2p E cath MTE n = 3 10p 0 mc 2 q MTE E cath Courtesy of I. Bazarov

24 New photocathode materials Thermal part still represents a substantial fraction of the emittance of the electron beam: Monte-Carlo code will be further developed to support layered or graded structures Engineering doping profiles of III-V semiconductors could lead to smaller MTEs Lifetime could still be an issue while we are pushing to higher average currents: Na 2 KSb will be soon tested in the photoinjector. S20 photocathode (Na 2 KSb:Cs) can provide higher QE at longer wavelength with reduced thermal emittance w.r.t. Bialkali antimonides. New alkali metal sources will be tested aiming at improving the growth reliability

25 New DC gun Higher brightness require larger fields at the cathode surface diagnostics beamline (new gun) We designed a segmented insulator with intermediate guard rings to catch any field emitted electrons before they reach the insulator material. new DC gun

26 Main Linac Cryomodule ~40K thermal shield HGRP support post HGRP Gate valve SC magnet & BPM 7-cell cavity Beamline HOM absorber Engineering and cryogenic design of ERL main linac cryomodule near completion: First CW, high beam current (100 ma) linac cryomodule Design of the cryomodule and of the cryogenic system reviewed: 1 st review in 2010, focused on conceptual cryomodule design: Endorsed conceptual design and found no major problems 2 nd review October 2011 focused on cryogenic system: 3 rd review October 2012 focused on final cryomodule design: Endorsed design and found no major problems Courtesy of M. Liepe

27 ERL R&D at Cornell for all that s needed for an ERL Return arc Turn-around B Turn-around A High-current SRF cryomodule will exist Gun - exists SRF injector - exists Cathode - exists Beam stop - exists Outline of a possible Cornell test ERL Many R&D Challenges. Foremost: Can an injector be made to deliver required emittance? Can the injector operate with acceptable current and lifetime? Can the main linac operate with acceptable Q 0 and HOM performance at 100 ma?

28 Conclusions Most of the R&D components needed for an ERL have reached or exceeded the specs Cornell photoinjector holds now the world record in terms of average current Beam emittance limited by gun accelerating gradient and thermal emittance If the beam could be accelerated to 5 GeV it might produce x-ray brighter than any ring today The photoinjector has already met the specification for at least one of the foreseen ERL modes of operation SRF 7-cell cavity prototype for the main linac cryomodule has been extensively tested demonstrating the world record in terms of Q 0 New DC gun, new photocathode materials and a full linac cryomodule will push even further the knowledge to realize an ERL based X-ray source facility.

29 Acknowledgements: S.A. Belomestnykh, D.H. Bilderback, M.G. Billing, J.D. Brock, B.W. Buckley, S.S. Chapman, E.P. Chojnacki, Z.A. Conway, J.A. Crittenden, D. Dale, J.A. Dobbins, R.D. Ehrlich, M.P. Ehrlichman, K.D. Finkelstein, E. Fontes, M.J. Forster, S.W. Gray, S. Greenwald, S.M. Gruner, D.L. Hartill, R.G. Helmke, A. Kazimirov, R.P. Kaplan, V.O. Kostroun, F.A. Laham, Y.H. Lau, Y. Li, X. Liu, F. Loehl, T. Miyajima, A. Meseck, A.A. Mikhailichenko, D. Ouzounov, H.S. Padamsee, S.B. Peck, M.A. Pfeifer, S.E. Posen, P.G. Quigley, P. Revesz, D.H. Rice, U. Sae-Ueng, D.C. Sagan, W. Schaff, J.O. Sears, V.D. Shemelin, C.K. Sinclair, D.M. Smilgies, E.N. Smith, K.W. Smolenski, Ch. Spethmann, C. Song, T. Tanabe, A.B. Temnykh, M. Tigner, N.R.A. Valles, V.G. Veshcherevich, Z. Wang, A.R. Woll, Y. Xie and in particular to I.V. Bazarov, A. Bartnik, B.M. Dunham,, C. Gulliford, G.H. Hoffstaetter, S.S. Karkare, M. U. Liepe, C.E. Mayes, J.M. Maxson, Z. Zhao For providing the material used in this presentation Funding agencies NSF (DMR ), DOE (DE-SC ) Thank you for the attention!