Comments from LCLS FAC Meeting (April 2004): J. Rößbach:“How do you detect weak FEL power when the gain is very low (few hundred)?” K. Robinson:“Can you modulate the laser heater in some way to help FEL signal detection?” Comments from LCLS FAC Meeting (April 2004): J. Rößbach:“How do you detect weak FEL power when the gain is very low (few hundred)?” K. Robinson:“Can you modulate the laser heater in some way to help FEL signal detection?” Laser-Heater Modulation P. Emma, Z. Huang, J. Wu

modulate laser power (1.2 to 19 7 Hz) Kem Robinson idea… modulated slice energy spread (0.01% to 0.04% rms at 14 GeV) laser heater Laser Heater at 135 MeV

Simulate LCLS (Linac + M. Xie) with linac jitter, large emittance (3  m), spontaneous radiation background, and laser-heater modulated at 7 Hz (or other unique frequency << 120 Hz) 1.5-Å LCLS, except  x,y = 3  m 2% rms charge jitter 0.5 ps rms gun-timing jitter 0.1-deg rms RF phase jitter (each of 4 linacs) 0.1% rms RF amplitude jitter (each of 4 linacs) 2% rms emittance jitter includes small emittance growth in laser-heater 100-MW spontaneous power (1% BW cut)* 10% rms radiation energy measurement noise fast, accurate 2 nd -order linac model in Matlab, with jitter * Sven Reiche says 10-MW at 1% BW cut (allows a 3.5-  m emittance)

Effects of Linac Jitter bunch length energy spread peak current e  relative energy bunch arrival time

P < P sat : P > P sat : 1-D startup power: Use ‘Ming Xie’ model to calculate gain length, L g, then calculate power as: Assume 1% BW cut to get 100-MW spontaneous power (none)

Red:Total power + meas. noise Magenta:Spontaneous power Green:FEL power  Gain   230

14.7 ± 4.1 m ± %  x,y = 3.0  m 4.2 ± 0.6 kA 0 ± % 1.0 ± 0.02 nC 21 ± 3.0  m

7 Hz FFT of total power + noise  Gain   230 Get similar result if P spont = 10 MW and  x,y = 3.5  m