1. 1 Daniel Ratner dratner@slac.stanford.edu 1 Gain Length and Taper August, 2009 FEL Gain length and Taper Measurements at LCLS D. Ratner A. Brachmann, F.J. Decker, Y. Ding, D. Dowell, P. Emma, J. Frisch, S. Gilevich, G. Hays, P. Hering, Z. Huang, R. Iverson, H. Loos, A. Miahnahri, H.D. Nuhn, J. Turner, J. Welch, W. White, J. Wu, D. Xiang, G. Yocky (SLAC, Menlo Park, California) W. M. Fawley (LBNL, Berkeley, California) August 25 th, 2009

2. 2 Daniel Ratner dratner@slac.stanford.edu 2 Gain Length and Taper August, 2009 Gain Length Measurement Goal of this talk: Characterize FEL as function of undulator length Gain length studies: Measure power, radius, position on YAG Exp. GAIN SATURATION Saturation studies: Measure energy loss of electrons with dump BPMs

3. 3 Daniel Ratner dratner@slac.stanford.edu 3 Gain Length and Taper August, 2009 Gain Length Measurement Moving Undulators: Measure power at YAG screen (50 m from last U) Remove undulators sequentially YAG Screen X-rays e-e- Problem: 3 minutes per undulator move If FEL unstable, too long to measure gain length Too long for a routine measurement Need faster method!

4. 4 Daniel Ratner dratner@slac.stanford.edu 4 Gain Length and Taper August, 2009 Gain Length Measurement Shut Off FEL Process All undulators remain inserted FEL Bunching misaligned, smeared Bunching from FEL process YAG Screen Full FEL Sequential dipole correctors kick beam ~ 15  rad Strongly suppress short wavelength FEL Smears bunching in direction of motion, decreases overlap with X-rays Full gain length scan in < 10 minutes

5. 5 Daniel Ratner dratner@slac.stanford.edu 5 Gain Length and Taper August, 2009 Gain Length Measurement Kick Method Problems Bunching from FEL process SASE re-start (New bunching) YAG Screen All undulators remain inserted FEL Spontaneous background is larger FEL Process can restart, produce additional spots If spots overlap, distorts measurement

6. 6 Daniel Ratner dratner@slac.stanford.edu 6 Gain Length and Taper August, 2009 Gain Length Measurement FEL restarts after kick Top image shows full FEL Bottom image shows second spot from distorted orbit Cropping image or second kick fix problem Full FEL X Kick at Undulator 5 X Kick +0.75 mm

7. 7 Daniel Ratner dratner@slac.stanford.edu 7 Gain Length and Taper August, 2009 Gain Length Measurement Move Undulator vs. Kick Beam

8. 8 Daniel Ratner dratner@slac.stanford.edu 8 Gain Length and Taper August, 2009 Short Wavelength Measurement Gain length of 3.3m at 13.6 GeV  x,y  0.4  m (slice) I pk  3.0 kA  E /E  0.01% (slice)

9. 9 Daniel Ratner dratner@slac.stanford.edu 9 Gain Length and Taper August, 2009 Long Wavelength Measurement Gain length at 4.7 GeV (1.3 nm) Kick suppression weak so use undulator-pull Stop before saturation to protect YAG GL: 1.62+/-0.15 m M. Xie: ~1.5 m  x,y  0.4  m (slice) I pk  1 kA  E /E  0.01% (slice)

10. 10 Daniel Ratner dratner@slac.stanford.edu 10 Gain Length and Taper August, 2009 Gain Length Measurement Laser Heater varies energy spread, 0 to 120 keV. Microbunching Instability? Laser heater improves gain length Parameters: 1.5Å, 250pC, 3kA, Compression factor = 90

11. 11 Daniel Ratner dratner@slac.stanford.edu 11 Gain Length and Taper August, 2009 Taper Studies Taper undulator K to match e - energy loss Linear Taper Saturation Taper Linear taper for wakefield and spontaneous radiation Saturation taper for FEL radiation Maximize final power by searching for optimal saturation taper

12. 12 Daniel Ratner dratner@slac.stanford.edu 12 Gain Length and Taper August, 2009 Taper Studies Use same method to study taper Dipole corrector kicks, measure electron E-loss (YAG saturates)

13. 13 Daniel Ratner dratner@slac.stanford.edu 13 Gain Length and Taper August, 2009 Taper Studies Saturation taper gives factor of 2 power gain Simulation more effective, but sensitive to distribution Gain factor of 2.4 from taper Gain factor of 3.3 from taper Experiment Simulation

14. 14 Daniel Ratner dratner@slac.stanford.edu 14 Gain Length and Taper August, 2009 Thanks to: LCLS project director J. Galayda, Commissioning Team and many collaborators and visitors from LBNL, LLNL, DESY

15. 15 Daniel Ratner dratner@slac.stanford.edu 15 Gain Length and Taper August, 2009

16. 16 Daniel Ratner dratner@slac.stanford.edu 16 Gain Length and Taper August, 2009 Gain Length Measurement Two spots from kick? X Kick

17. 17 Daniel Ratner dratner@slac.stanford.edu 17 Gain Length and Taper August, 2009 Longer wavelength measurement Two example gain length measurements Why pull undulator 1.62+/-0.15 m gain length 2.85+/-0.06 m gain length 4.7 GeV (Undulator pull)13.6 GeV (X-corr Kick) Stop before saturation to protect YAG  x,y  0.4  m (slice) I pk  1.5 kA  E /E  0.02% (slice)  x,y  0.4  m (slice) I pk  5.0 kA  E /E  0.01% (slice)