EMITTANCE GROWTH DUE TO WAKE FIELDS ON CURVED BUNCH TRAJECTORIES

Very short bunches with high peak currents are required in linac based X-ray Free Electron Lasers (SASE-FELs) and in the next generation of linear colliders. In most designs, short bunches are produced by longi tudinal compression in magnet chicanes, where particles with different energies have different path lengths so that a bunch with an energy d istribution correlated with longitudinal particle position can shrink in length. However, care has to be taken that the low emittance beams are not blown up by wake field effects. Especially the wake field calc ulation for very short bunches on curved trajectories with relatively small bending radii (of the order of meters) requires consideration of ''overtaking effects''. ''Overtaking'' means, that on a curved trajec tory the wake of the bunch tail may reach the head because the field t ravels on a shorter straight path. We present a treatment to calculate these fields considering transient regimes and shielding effects as w ell as calculations on the emittance growth for the case of the TTF-FE L bunch compression system.