1. Brookhaven Science Associates U.S. Department of Energy A Few Comments on the Photoinjector Performance X.J. Wang National Synchrotron Light Source Brookhaven National Laboratory Upton, NY 11973, USA Presented at the UCLA High-Power Brightness Beams Workshop Nov.9, 2004

2. Brookhaven Science Associates U.S. Department of Energy Acknowledgement I would like to thank many colleague who have educated me over many years on various subjects I mentioned here, BNL: M. Babzien, K. Batchelor, I. Ben-Zvi, X.Y. Chang, A. Doyuran, J. Fisher, W. Graves, H. Loss, J. Murphy, T. Rao, J. Rose, B. Sheehy, J. Sheen, Z. Wu, V. Yakimenko and L.H. Yu ANL: S. Biedron, M. Conde, W. Gai, J. Lewellen, S. Milton, J. Power SLAC: R. Miller, D.T. Palmer Japan: A. Endo, K. Kobayashi, F. Sakai, J. Urakawa, Uesaka and M. Washio And Many others. Thank you!

3. Brookhaven Science Associates U.S. Department of Energy BNL NSLS PERL Gun Workshop (1/2001) High QE Green or Red Cathode. 2 ½ Cell low temperature RF gun DC and RF gun both capable of CW operation. Dc: 10 mm-mrad RF: 1.0 mm-mrad

4. Brookhaven Science Associates U.S. Department of Energy Challenges in High-Brightness Electron Source R&D Stability and Reliability Timing jitter and its control Better Theoretical Understanding Thermal Emittance – fundamental limit and importance of beam instrumentation Improve performance – 6-D optimization Next Generation Electron source: 1. CW injector – DC, RF, SRF, what should be? 3H - Heat, Heat and Heat; 2. Brighter sources - Higher field gun, pulse DC gun, laser plasma source and others.

5. Brookhaven Science Associates U.S. Department of Energy High Brightness Electron Injectors 500 kV Spring-8 DC Injector How to create the Greenfield FEL injector? Optimize 6-D phase space, not just  n or I p Optimize 6-D phase space, not just  n or I p To realize this the GFEL injector should achieve: To realize this the GFEL injector should achieve: G > 500 MV/m, E > 50 MeV in order to produce and preserve the beam. TypeDC GunRF GunGreenField 50 500 E [MeV]0.55 50 G[MV/m]10100 500  [ps]50010<1 I p [A]10100500 Q [nC]0.51<0.5  n [  m]110.1 BNL RF Photoinjector

6. Brookhaven Science Associates U.S. Department of Energy

7. XFEL vs Linear Collider M. Brandin et al, CLIC Note 543 (2002)

8. Brookhaven Science Associates U.S. Department of Energy ISSUES Introduction – What are the performance and Applications Vacuum and QE do Matter. 6-D Performance Optimization Thermal Emittance Timing Jitter – What is required? Theoretical Challenges of RF Gun Femto-second Kilo-Ampere Electron Beam Generation - Longitudinal Emittance Compensation

9. Brookhaven Science Associates U.S. Department of Energy They all driven by a photocathode RF Gun Based Linac

10. Brookhaven Science Associates U.S. Department of Energy Introduction All the FEL reach saturation does not test the limit of the emittance performance: 1. LEUTL and TTF I < 6-10 mm-mrad 2. VISA <2.0 - 2.5 mm-mrad 3. DUV-FEL < 5 mm-mra Does Any Physics Experiment Test the limit: 1. Laser Compton Scattering < 2 mm-mrad 2. IFEL Micro-bunching - Stella Experiment < 1-2 mm-mrad

11. Brookhaven Science Associates U.S. Department of Energy Why Photocathode RF Gun 6-D performance – smaller emittance and shorter bunch. Flexibility. But it bring more issues, mainly laser and cathode: Stability Reliability Uniformity – QE, transverse and longitudinal distribution Jitters – position and time

12. Brookhaven Science Associates U.S. Department of Energy Vacuum and QE Do Matter

13. Brookhaven Science Associates U.S. Department of Energy Laser-Induced Explosive Emission (X.J. Wang et al, J. Appl. Phys. 72(3), 888-894 (1992))

14. Brookhaven Science Associates U.S. Department of Energy What We would like Photoinjector do Programmable in both transverse and longitudinal distribution No timing jitter No energy fluctuation Perfect point stability 7/24 available Remote controllable NO laser physicist. rmspeak Timing jitter50 - 100, fs200 - 400, fs energy1,%5,% Point stability0.25,1,% Transverse uniformity 2.5,%10,%

15. Brookhaven Science Associates U.S. Department of Energy Photo-injector Beam Diagnostics Energy Charge RF Gun Phase

16. Brookhaven Science Associates U.S. Department of Energy Quantum Efficiency Measurements

17. Brookhaven Science Associates U.S. Department of Energy Stability and Reliability Leads To Better Performance

18. Brookhaven Science Associates U.S. Department of Energy Thermal Emittance 00.20.40.60.811.2 0 0.2 0.4 0.6 0.8 1 1.2 Horizontal RMS laser size (mm)  N (mm-mrad) Electrons are emitted with a kinetic energy E k laser spot assumed uniform with radius r Example of measurement for Cu-cathode (Courtesy of W. Graves) Linear fit gives E k =0.43 eV Nonlinear fit gives  rf =3.1+/-0.5,  cu =4.73+/-0.04 eV, and E k =0.40 eV ICFA/BD Sardinia July 2002

19. Brookhaven Science Associates U.S. Department of Energy Mg thermal Emittance

20. Brookhaven Science Associates U.S. Department of Energy Slice Emittance

21. Brookhaven Science Associates U.S. Department of Energy RF Photoinjector Theory Are all emittance uncorrelated? K-J. ’ s theory: Emittnace growth (Rieser):

22. Brookhaven Science Associates U.S. Department of Energy 1300 MHz E b = 15-20 MeV I macro = 100-400 mA Q = 1-4 nC  rms = 1.6 mm-mrad  = 0.2% Injection  = 30 o Solenoid = 300A Bucking Sol. = 310A The Advanced FEL Photoinjector Operates at 20 MV/m Gradient and 200 mA Average Current (D. Nguyen’s talk at BNL PERL workshop, Jan 2001)

23. Brookhaven Science Associates U.S. Department of Energy

25. Emittance Optimization at 45 MeV

26. Brookhaven Science Associates U.S. Department of Energy

27. Timing jitter effects – photocathode RF gun Jitter smaller < 200 fs (rms)

28. Brookhaven Science Associates U.S. Department of Energy Longitudinal Emittance Compensation Phys. Rev. E. 54, R3121 (1996) PAC 97

29. Brookhaven Science Associates U.S. Department of Energy Linac 2000)

30. Brookhaven Science Associates U.S. Department of Energy Buncher head tail

31. Brookhaven Science Associates U.S. Department of Energy 20 pC at 40 MeV Presented at the linac 2000

32. Brookhaven Science Associates U.S. Department of Energy 10 fs kilo-Ampere Electron beam generation 10. 8 fs

33. Brookhaven Science Associates U.S. Department of Energy

34. Summary of Kilo-Ampere Electron Beam X.J. Wang and X.Y. Chang, NIMA 507 (2003) 310 – 313