Regenerative FEL amplifier at the TESLA test facility at DESY

This paper presents a conceptual design of a regenerative FEL amplifier (RAFEL) as an extension of the single-pass free electron laser project at the TESLA test facility (TTF) at DESY. The proposed scheme requires the additional installation of only two optical components for a narrow-band feedback system and is fully compatible with the present design and the infrastructure developed for the TTF FEL project. It would allow to construct a tunable VUV laser with a minimum wavelength around 60 nm, a pulse duration of about 1 ps, a peak power of about 300 MW and an average power of about 25 W. The output radiation of the regenerative FEL amplifier would possess all the features which are usually associated with laser radiation: full transverse and longitudinal coherence and shot-to-shot stability of the output power. The degeneracy parameter of the output radiation would be about 1014 and thus have the same order of magnitude as that of a quantum laser operating in the visible