LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 1 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Measuring the FEL gain with trajectory distortion Z. Huang, SLAC January 20, 2004

LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 2 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Introduction  Stop the FEL amplification in the undulator and measure the radiation properties at the end (TTF results, PRL, 2001)

LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 3 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Single Kick Error (SKE)  Two effects of SKE (Tanaka et al., FEL2003): reduction in coupling smearing of microbunching  Critical angle to destroy FEL interaction

LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 4 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Trajectory distortion in LCLS  Offset the quad by Q x amount. If the focal length of the quad is F, then the kick angle is  Q = -Q x /F  Set  Q =  c, since F ≈ 10m, Q x ≈ 60  m, so that the maximum betatron amplitude  x  Q ≈ 2Q x

LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 5 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center GENESIS simulation results  Total power still grows due to interaction with higher-order modes & spontaneous radiation (div. angle up to 100  rad)  On-axis power is dominated by the FEL fundamental mode only (~1  rad) and stops growing when Q x ~60  m.

LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 6 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Z-dependence  Initial stage: no microbunching to destroy, still some weak interaction + a lot of spontaneous  Exponential growth region, works reasonably well Qx = 60  m Vary Qx

LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 7 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Summary  It worked (as in TTF,…)  Trajectory distortion can stop further growth of FEL fundamental mode, but other higher-order modes (including spontaneous) increase the background  Collimation helps to pick up on-axis signal, or can we find the hot spot by data analysis?  Measurement results should be cross-checked with other methods, if possible

LCLS undulator diagnostics and commissioning workshop January 19-20, 2004 (UCLA) Zhirong Huang, SLAC 8 Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center  Spontaneous fundamental radiation within 3% bandwidth reaches 60 MW, FEL signal can dominate after 40m Spontaneous v.s. FEL gain Spontaneous fundamental in 3% BW (Z. Huang, K.-J. Kim FEL2002)