Beam dynamics of the 250 MEV PSI XFEL Injector

The PSI XFEL project aims at developing a pulsed high- brightness, high-current electron source which is one of the prerequisites of a cost-effective high-power laser-like X- ray light source. Creating an ultra low emittance beam is a great challenge, transporting, i.e., accelerating and compressing it is equally difficult. We present a 3D start-to-end simulation of our planned 250 MeV injector facility. The injector consists of a photocathode with pulsed DC acceleration followed by a two-cell standing-wave L-band cavity that leads into a ballistic bunching section. After some further velocity bunching in an L-band structure the electron beam enters several S-band structures which accelerate it up to the final energy of 250 MeV. An X-band RF structure prepares the beam for the following bunch compressor in which the target peak current of 350 A is reached. The target value of the slice emittance is 0.1 mm mrad, necessitating precise beam dynamics simulations. For the 3D simulations we use IMPACT-T, a time domain parallel particle tracking code, in which the self fields are treated in the electrostatic approximation. We discuss various issues such as projected and slice emittance preservation and shed light on some of the differences between an envelope and the 3D model.