  Since spring, 2005 vacuum ultraviolet free electron laser (VUV FEL) at DESY
operates as user facility in the wavelength range around 30 nm. Electron beam
formation system at the VUV FEL is essentially nonlinear and naturally results
in a formation of a short high-current leading peak in the density distribution
that produces FEL radiation. The main feature of the femtosecond mode of
operation of the VUV FEL is the production of short, down to 20 fs radiation
pulses with GW-level peak power. This paper is devoted to detailed studies of
the short-pulse effects in the VUV FEL at DESY operating in the femtosecond
regime, and to analysis of first experimental results.
