Ultra-short bunches by using a quasi-continuous compressor scheme in a long beam transfer line

Integration of an X-ray free electron laser facility is planned in the TESLA design for a future e/sup +/e/sup -/ linear collider. In the present scheme, the FEL drive beam is transported to a central user facility in a several km long beamline. In this paper we present how the longitudinal wakefield effect for short bunches in combination with a special beamline magnetic lattice can be used to obtain compression down to an rms-bunch length in the range of a few /spl mu/m. A peak current of more than 50kA would enable the X-ray FEL facility to generate photon wavelengths in the sub-Angstrom regime.