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, J. Welch, W. White, J. Wu, D. Xiang, G. Yocky (SLAC, Menlo Park, California) W. M. Fawley (LBNL, Berkeley, California) August 25 th, 2009 Gain Length and Taper August, 2009 1 1 Daniel Ratner dratner@slac. stanford. edu

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

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 YAG Need faster method! Screen X-rays e. Gain Length and Taper August, 2009 3 3 Daniel Ratner dratner@slac. stanford. edu

Gain Length Measurement Shut Off FEL Process Sequential dipole correctors kick beam ~ 15 mrad 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 Bunching from FEL process Gain Length and Taper August, 2009 YAG Screen Bunching misaligned, smeared All undulators remain inserted 4 4 Daniel Ratner dratner@slac. stanford. edu

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

Gain Length Measurement Full 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 +0. 75 mm Gain Length and Taper August, 2009 6 6 Daniel Ratner dratner@slac. stanford. edu

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

Short Wavelength Measurement Gain length of 3. 3 m at 13. 6 Ge. V gex, y 0. 4 mm (slice) Ipk 3. 0 k. A s. E/E 0. 01% (slice) Gain Length and Taper August, 2009 8 8 Daniel Ratner dratner@slac. stanford. edu

Long Wavelength Measurement Gain length at 4. 7 Ge. V (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 k. A s. E/E 0. 01% (slice) Gain Length and Taper August, 2009 9 9 Daniel Ratner dratner@slac. stanford. edu

Gain Length Measurement Laser Heater varies energy spread, 0 to 120 ke. V. Microbunching Instability? Laser heater improves gain length Parameters: 1. 5Å, 250 p. C, 3 k. A, Compression factor = 90 Gain Length and Taper August, 2009 10 10 Daniel Ratner dratner@slac. stanford. edu

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 Gain Length and Taper August, 2009 11 11 Daniel Ratner dratner@slac. stanford. edu

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

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 Gain Length and Taper August, 2009 13 13 Daniel Ratner dratner@slac. stanford. edu

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

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

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

Longer wavelength measurement Two example gain length measurements Why pull undulator 4. 7 Ge. V (Undulator pull) 13. 6 Ge. V (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 k. A s. E/E 0. 02% (slice) Gain Length and Taper August, 2009 Ipk 5. 0 k. A s. E/E 0. 01% (slice) 17 17 Daniel Ratner dratner@slac. stanford. edu