Weak FEL Gain Detection with a Modulated Laser-Based Beam Heater

For an x-ray free-electron laser (FEL) such as the LCLS, the FEL gain signal is accompanied by spontaneous radiation with a significant power level. Detecting the weak FEL gain among the large spontaneous background in the early stage of the exponential growth or for a low quality electron beam is important in commissioning the FEL. In this paper, we describe a simple “lock-in” method of weak FEL gain detection, suggested by K. Robinson, accomplished by slowly modulating the laser power of a designated beam heater that controls the local energy spread of the electron beam. We present numerical modelling that shows the effectiveness of this method and discuss its implementation in the LCLS.