A 3D particle tracking technique for FEL start-up and saturation effects

Self-consistent simulation of a LINAC-driven FEL by time-domain particle tracking can give very detailed results on both the produced radiation and the evolution of the electron bunch. We show that when special subsets are tracked, instead of individual macro-particles, only a few of these subsets are required to obtain converging results. The subsets used are short longitudinal arrays of macro-particles, of the order of a few ponderomotive waves, distributed longitudinally in such a way that they are almost only sensitive to stimulated emission. This new approach has been carried out with the 3D General Particle Tracer code and a set of axisymmetric Gaussian waves propagating in free space. Due to the first-principles approach, it can be used for a variety of radiation problems, including studies of FEL start-up and saturation effects. The model and two applications will be presented.