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FEL Gain length and Taper Measurements at LCLS

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,

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FEL Gain length and Taper Measurements at LCLS

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  1. 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 25th, 2009

  2. Gain Length Measurement • Goal of this talk: Characterize FEL as function of undulator length Exp. GAIN SATURATION Gain length studies: • Measure power, radius, position on YAG Saturation studies: • Measure energy loss of electrons with dump BPMs

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

  4. Gain Length Measurement • Shut Off FEL Process • Sequential dipole correctors kick beam ~ 15mrad • Strongly suppress short wavelength FEL • Smears bunching in direction of motion, decreases overlap with X-rays • Full gain length scan in < 10 minutes Full FEL FEL Bunching from FEL process YAG Screen Bunching misaligned, smeared All undulators remain inserted

  5. Gain Length Measurement • Kick Method Problems • Spontaneous background is larger • FEL Process can restart, produce additional spots • If spots overlap, distorts measurement FEL SASE re-start (New bunching) FEL Bunching from FEL process All undulators remain inserted YAG Screen

  6. Gain Length Measurement Full FEL • FEL restarts after kick • Top image shows full FEL • Bottom image shows second spot from distorted orbit • Cropping image or second kick fix problem X Kick at Undulator 5 X Kick +0.75 mm

  7. Gain Length Measurement • Move Undulator vs. Kick Beam

  8. Short Wavelength Measurement • Gain length of 3.3m at 13.6 GeV gex,y 0.4 mm (slice) Ipk 3.0 kA sE/E 0.01% (slice)

  9. 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 gex,y 0.4 mm (slice) Ipk 1 kA sE/E 0.01% (slice)

  10. 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. Taper Studies • Taper undulator K to match e- energy loss • Linear taper for wakefield and spontaneous radiation • Saturation taper for FEL radiation Linear Taper Saturation Taper Maximize final power by searching for optimal saturation taper

  12. Taper Studies • Use same method to study taper • Dipole corrector kicks, measure electron E-loss (YAG saturates)

  13. Taper Studies • Saturation taper gives factor of 2 power gain • Simulation more effective, but sensitive to distribution Experiment Simulation Gain factor of 2.4 from taper Gain factor of 3.3 from taper

  14. Thanks to: LCLS project director J. Galayda, Commissioning Team and many collaborators and visitors from LBNL, LLNL, DESY

  15. Gain Length Measurement • Two spots from kick? X Kick X Kick

  16. Longer wavelength measurement • Two example gain length measurements • Why pull undulator 4.7 GeV (Undulator pull) 13.6 GeV (X-corr Kick) Stop before saturation to protect YAG 1.62+/-0.15 m gain length 2.85+/-0.06 m gain length gex,y 0.4 mm (slice) Ipk 1.5 kA sE/E 0.02% (slice) gex,y 0.4 mm (slice) Ipk 5.0 kA sE/E 0.01% (slice)

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