1. D.H. Dowell/MIT Talk, May 31, 2002 1 David H. Dowell Stanford Linear Accelerator Center Photocathode RF Guns and Bunch Compressors for High-Duty Factor FELs Introduction Architecture of SASE FELs RF Gun Technologies Example: A Low-Frequency, High-Duty Factor RF Photoinjector at 433 MHz Bunch Compressor Physics Summary and Conclusion

2. D.H. Dowell/MIT Talk, May 31, 2002 2 Architectures of the SASE X-Ray FEL Single Pass, Normal Conducting => Leutl, VISA(ATF), SDL, LCLS RF Gun Accelerator Bunch Compressor Accelerator Long Undulator X-Rays High Energy E-beam Dump 3 rd Harmonic Linearizer Energy Recovered Linac (ERL), SRF => JLab, Cornell RF Gun SRF Accelerator Low Energy E-beam Dump Long Undulator X-Rays Bend/ Compressor Bend/ De-Compressor Accelerator SRF Accelerator Bunch Compressor 3 rd Harmonic Linearizer

3. D.H. Dowell/MIT Talk, May 31, 2002 3 RF Gun Technologies for the Two SASE Architectures Single Pass, Normal Conducting: Energy Recovered Linac (ERL), SRF: Low Duty Factor: Single microbunch at 1 to 120 Hz, 0.1 to 1 nC / bunch S-band (2856MHz), 1.6 cell, BNL Gun, ~100 MV/m cathode field Metal Cathodes: Cu, Mg UV Drive Laser: Freq. Quadrupled Nd:Glass, Ti-Sapphire; 2-10 ps(fwhm) High Duty Factor: CW, microbunches at 10 to 70 MHz, 0.05 to 1 nC / bunch DC, 433 MHz, L-band (1500&1300MHz) Guns: 10 to 30 MV/m cathode field Semi-Conductor Cathodes: GaAs, CsTe, CsKSb Visible Wavelength Drive Laser: Frequency Doubled Nd:YLF High Duty Factor requires low-frequency gun with semi-conductor cathode Build upon experience gained from high power RF gun development at Boeing & LANL

4. D.H. Dowell/MIT Talk, May 31, 2002 4 Example of Demonstrated Technology: A Low-Frequency, High-Duty Factor 433 MHz RF Photoinjector Developed by Boeing and Los Alamos

5. D.H. Dowell/MIT Talk, May 31, 2002 5 Historical Perspective Motivation: Design, build and test an RF photocathode gun capable of operating at high current and high duty factor. Result: A 1992 demonstration of a two-cell, 433 MHz photocathode gun at 32 mA of average current and 25% duty factor.

6. D.H. Dowell/MIT Talk, May 31, 2002 6 Photoinjector Design Philosophy Use a CW low frequency photocathode gun to generate high charge (1-5 nC) and long (50 ps) micropulses. Advantages: Capable of CW operation High charge Long micropulses Disadvantage: Cathode field limited to 25-30 MV/m Accelerate in Low frequency RF cavities. Advantages: Minimizes wakefields CW operation Disadvantage: Accelerating gradient limited to 5 MV/m Linearize and compress to high peak current at 20 MeV or higher. Advantages: Linearizing improves compression Reduces space charge emittance growth Disadvantage: Emittance growth due to coherence synchrotron radiation Excellent Beam Quality at High Beam Current

7. D.H. Dowell/MIT Talk, May 31, 2002 7 RF Gun 433 MHz Booster Accelerator Section 433 MHz Longitudinal Linearizer 1.3 GHz Bunch Compressor Main Accelerator 1.3 GHz K 2 SbCs Cathode PhotoInjector Layout of the 433 MHz PhotoInjector

8. D.H. Dowell/MIT Talk, May 31, 2002 8 2 MeV Electron Beam 527 nm Drive Laser Beam CsKSb Photocathode RF Cavities Defocusing and Focusing RF Lenses Focusing Injector Coil Electron Beam Optics of the 433 MHz Photocathode Gun Cathode B-field bucking coil

9. D.H. Dowell/MIT Talk, May 31, 2002 9 The Boeing 433 MHz RF Photocathode Gun

10. D.H. Dowell/MIT Talk, May 31, 2002 10 Photocathode Performance: Photosensitive Material:K 2 CsSb Multialkali Quantum Efficiency:5% to 12% Peak Current:45 to 132 amperes Cathode Lifetime:1 to 10 hours Angle of Incidence:near normal incidence Gun Parameters: Cathode Gradient:26 MV/meter Cavity Type:Water-cooled copper Number of cells:4 RF Frequency:433 x10 6 Hertz Final Energy:5 MeV(4-cells) RF Power:600 x10 3 Watts Duty Factor:25%, 30 Hertz and 8.3 ms Laser Parameters: Micropulse Length:53 ps, FWHM Micropulse Frequency:27 x10 6 Hertz Macropulse Length:10 ms Macropulse frequency:30 Hertz Wavelength:527 nm Cathode Spot Size:3-5 mm FWHM Temporal and Transverse Distribution:gaussian, gaussian Micropulse Energy:0.47 microjoule Energy Stability:1% to 5% Pulse-to-pulse separation: 37 ns Micropulse Frequency:27 x10 6 Hertz Gun Performance: Emittance (microns, RMS):5 to 10 for 1 to 7 nCoulomb Charge:1 to 7 nCoulomb Energy:5 MeV Energy Spread:100 to 150 keV Demonstrated Performance of 433 MHz Photocathode Gun, 1992 H-D Test

11. D.H. Dowell/MIT Talk, May 31, 2002 11 Types of Photocathodes MaterialQE Range Drive Laser Wavelength Cathode Fab. Vacuum Req. Drive Laser Metal~0.02-0.06%260 nm, UVNone10 -7 T Difficult (Cu, Mo…) CsK 2 Sb 10-14%527 nmDifficult10 -10 T Moderate CsTe 10-14%260 nmEasy10 -9 T Moderate to Difficult LaB 6 ~0.1%355 nmEasy10 -7 T Difficult Ga As (Cs) 1-5%527 nmModerate10 -11 T Moderate

12. D.H. Dowell/MIT Talk, May 31, 2002 12 Semi-Conductor Photocathode Fabrication Chamber Vacuum Valve Connection to Gun Cavity QE Measurement Laser (GreNe) RGA Head Thin Film Monitor Sb, K, Cs Sources N 2 Inlet/ Outlet 2 meter Cathode Stick

13. D.H. Dowell/MIT Talk, May 31, 2002 13 0 2 4 6 8 10 12 14 16 6/12/957/12/95 10/24/9511/14/9512/13/95 1/3/96 1/23/96 2/1/96 2/13/962/28/963/19/96 5/2/96 5/14/965/23/96 6/3/96 Fabrication Date Quantum Efficiency (%) 0 1 2 3 4 5 6 7 8 9 10 01234 QE x Drive Laser (% microjoules) Accelerated Micropulse Charge (nC) 4.3 x 3.8 mm FWHM 2.8 x 2.7 mm FWHM 4.386 QE x E laser x 0.72 QE Fabrication History and Gun Space Charge Limits

14. D.H. Dowell/MIT Talk, May 31, 2002 14 0.1 1 10 100 1000 10000 100000 1.00E-121.00E-111.00E-101.00E-09 WATER PARTIAL PRESSURE (TORR) 1/e LIFETIME (HOURS) Fabrication Chamber RF Cavities (original-vacuum) Least Squares Fit RF Cavities (improved-vacuum) Cathode Lifetime Vs. H 2 O Partial Pressure

15. D.H. Dowell/MIT Talk, May 31, 2002 15 Photocathode 1/e Lifetime Vs. Duty Factor 0.1 1 10 00.050.10.150.20.250.3 Duty Factor 2.3 Hour Lifetime 1/e Lifetime (Hours)

16. D.H. Dowell/MIT Talk, May 31, 2002 16 Cathode Rejuvenation and Improving Lifetime by Operating with Hot Cathode Rejuvenating a used K 2 CsSb cathode by heating it to 120 degrees C. The quantum efficiency increases at the rate of 2.5% / hour Photocathode quantum efficiency at elevated temperature in the RF cavity vacuum D.H. Dowell et al., NIM A356(1995)167-176

17. D.H. Dowell/MIT Talk, May 31, 2002 17 Heating the Cathode With a High Power Diode Laser 2 MeV Electron Beam 527 nm Drive Laser Beam K 2 CsSb Photocathode RF Cavities Cathode B-field bucking coil 800 nm Heater Laser Beam

18. D.H. Dowell/MIT Talk, May 31, 2002 18 Drive Laser Configuration Used in 1992 High Duty Test

19. D.H. Dowell/MIT Talk, May 31, 2002 19 433 MHz Gun Transverse Beam Quality Measurements 1992 and 1994-1996 Test Results

20. D.H. Dowell/MIT Talk, May 31, 2002 20 PARMELA_B Simulations at 0.5 nC PARMELA_B calculations provided by B. Koltenbah, Boeing

21. D.H. Dowell/MIT Talk, May 31, 2002 21 Bunch Compressor Physics

22. D.H. Dowell/MIT Talk, May 31, 2002 22 Non-Linearities in Bunch Compression Long microbunches are distorted in longitudinal phase space due to wakefields and RF curvature. 433 MHz cavities introduce minimal wakes, but still cause significant curvature. Introduce a RF section at third harmonic (1300 MHz) to cancel curvature of 433 MHz booster. Magnetic Pulse Compression Using a Third Harmonic RF Linearizer D.H. Dowell, T.D. Hayward and A.M. Vetter, Proceedings of the 1995 PAC, pp.992-994.

23. D.H. Dowell/MIT Talk, May 31, 2002 23 433 MHz Accelerator 1300 MHz Linearizer Chicane Buncher Streak Camera Quad Triplet Quad Triplet Quad Triplet Emittance Measurements Beam Dump The 20 MeV RF Photoinjector Demonstration Photocathode Gun Linearized energy programming for buncher To high voltage accelerator

24. D.H. Dowell/MIT Talk, May 31, 2002 24

25. D.H. Dowell/MIT Talk, May 31, 2002 25 Cooling and RF Feed for 433 MHz 5-Cell Section

26. D.H. Dowell/MIT Talk, May 31, 2002 26 3-Cell and 5-Cell APLE Cavity Booster 3-Cell Accelerator Cavity 5-Cell Accelerator Cavities

27. D.H. Dowell/MIT Talk, May 31, 2002 27 1300 MHz (third harmonic) energy spectrum programming for bunch compression Three dipole magnetic buncher and diagnostics 1300 MHz Linearizer and Three-Dipole Chicane Compressor

28. D.H. Dowell/MIT Talk, May 31, 2002 28 Boeing Chicane Compressor Achromatic chicane composed of three n=1/2 dipoles. 30 o 60 o 30 o 19.5 o 384 mm 600 mm

29. D.H. Dowell/MIT Talk, May 31, 2002 29 H T 11 22 HT H T Time Linearizer RF Waveform 433 MHz RF Waveform 20 MeV 9 = 2.2 Mev Pulse compression occurs at two linearizer phases, but the pulse is linearized only at the decelerating phase and at 1/9 the 433 MHz RF field.

30. D.H. Dowell/MIT Talk, May 31, 2002 30 Bend Plane Emittance Growth During Pulse Compression

31. D.H. Dowell/MIT Talk, May 31, 2002 31 Coherent Synchrotron Radiation Induced Emittance Growth Tail Radiation Electron Microbunch Traveling in an Arc CSR occurs when the bending of a relativistic electron beam allows the synchrotron radiation emitted by the tail of the microbunch to "catch up" with the head electrons. If the arc length of the bend is long enough, this radiation sweeps along the entire length of the microbunch and transfers energy from the tail to the head. Therefore CSR tends to increase the energy of the head while lowering that of the tail. Ref: Y.S. Derbenev et al., DESY TESLA-FEL Technical Note 95-05(1995)

32. D.H. Dowell/MIT Talk, May 31, 2002 32 -505101520 -50 0 50 Energy Loss Gradient (keV/m) Charge Distribution z/  z sb/  z =.25 sb/  z =1 sb/  z =4 sb/  z =8sb/  z =15 Transient CSR Radiation at the Magnetic Field Boundary Ref: D.H. Dowell and P.G. O’Shea, “Coherent Synchrotron Radiation Induced Emittance Growth in a Chicane Buncher”, contribution to PAC’97.

33. D.H. Dowell/MIT Talk, May 31, 2002 33 See also recent work by P. Emma and M. Borland

34. D.H. Dowell/MIT Talk, May 31, 2002 34 Summary and Conclusion Architectures of Single-Pass and Energy Recovery SASE FELs RF Photocathode Injector Design Philosophy/Approach: Gun, Booster, Linearizer and Compressor Review of high duty gun technology at 433 MHz Cathode Lifetime Cathode Fabrication Drive Laser RF Design Bunch compressor physics Coherent synchrotron radiation Technology developed in 1990’s is directly applicable to the new generation of Energy Recovered Linac SASE FELs