Wake Field Calculation for the Ttf-fel Bunch Compressor Section

The TTF Free Electron Laser needs very short bunches to produce self-amplified spontaneous emission (SASE). This short bunch length is produced in a magnetic bunch compressor where the trajectories of particles with different energy have different path length in a way that the bunch is longitudinally compressed. As a parasitic effect the wake fields produced by the passing bunch will have the possibility to interact with the bunch itself and cause emittance growth. The high frequency behaviour of the beam pipe in the bunch compressor has to be analysed in order to identify trapped higher order modes (HOM) and to estimate beam distortion. Because of the complexity of the bunch compressor section direct eigenmode calculation is not possible due to lack of available computer power. A technique is presented which allows to compute eigenmodes of rf-structures by using scattering-parameters of subsections of the bunch compressor. This is done numerically based on the computer code MAFIA to model the different sections of the beam pipe.