Optimal beam optics in the TTF-FEL bunch compression sections: minimizing the emittance growth

In the bunch compressing sections of the Tesla Test Facility Free Electron Laser, short bunches travel on trajectories with small bending radii. Thus, coherent synchrotron radiation (CSR) will play a significant role in beam dynamics. The energy loss of the bunch will vary longitudinally as well as transversally across the bunch and will induce an emittance growth. This emittance growth will affect the projective as well as the slice emittance (i.e., the emittance of sub-ensembles of particles with equal longitudinal position). The computer code TraFiC⁴ is a significantly augmented version of WAKE, which calculates these effects from first principles. One result of simulations is the observation that the slice emittance growth depends crucially on the initial Twiss parameters chosen for the beam. We discuss the mechanisms leading to this dependence. Simulation results for a range of optics will be given