TESLA FEL 2004-06 July 2004 Expected properties of the radiation from VUV-FEL at DESY (femtosecond mode of operation)

For the next three years the nominal ”long pulse” (200 fs) mode of FEL operation at VUVFEL, based on a linearized bunch compression, is not available due to the lack of a key element – a 3rd harmonic RF cavity. Essentially nonlinear compression leads naturally to a formation of a short high-current leading peak (spike) in the density distribution that produces FEL radiation. Such a mode of operation was successfully tested at VUV-FEL, Phase I. In this paper we present optimized parameters of the beam formation system that allow us to get a current spike which is bright enough to get SASE saturation for the VUV-FEL, Phase 2 at shortest design wavelength down to 6 nm. The main feature of the considered mode of operation is the production of short (15-50 fs FWHM) radiation pulses with GWlevel peak power that are attractive for many users. Main parameters of the SASE FEL radiation (temporal and spectral characteristics, intensity distributions, etc.) are presented, too. Preprint submitted to TTF FEL team and potential users 22 July 2004