Beam dynamics simulations for linacs driving short-wavelength FELs

The fast code HOMDYN has been recently developed, in the framework of the TTF (TESLA Test Facility) collaboration, in order to study the beam dynamics of linacs delivering high brightness beams as those needed for short wavelength FEL experiments. These linacs are typically driven by radio-frequency photo-injectors, where correlated time dependent space charge effects are of great relevance: these effects cannot be studied by standard beam optics codes (TRACE3D, etc.) and they have been modeled so far by means of multi-particle (PIC or quasistatic) codes requiring heavy cpu time and memory allocations. HOMDYN is able to describe the beam generation at the photo-cathode and the emittance compensation process in the injector even running on a laptop with very modest running times (less than a minute). In this paper we show how this capability of the code is exploited so to model a whole linac up to the point where the space charge dominated regime is of relevance (200 MeV)