Simulation of the SASE FEL: the PAL-XFEL

The X-ray free electron laser (FEL) based on the self-amplified spontaneous emission (SASE) scheme is now one of the most important subjects in the light source community. Although its theoretical understanding has seen a big progress, a simulation code is the ultimate tool for studying the SASE process. This paper simulates an example of the SASE FEL, focusing on the process leading to the saturation and beyond. The synchrotron oscillation is shown to be responsible for the behavior of SASE in the non-linear region. The difference from the conventional FEL amplifier is discussed. It is shown that the saturation point and the linear region's endpoint are defined unambiguously by use of the bunching factor.