The Bessy Soft X-Ray Fel User Facility

Free Electron Lasers will be the future light sources to generate intense ultra-short photon pulses. The user requests for an optimized 2^ndgeneration FEL facility in the VUV to soft X-ray range demands for ultra short photon pulses (Δt • 20 fs) at a peak power of several GW. Achieving a high shot to shot reproducibility of the pulse shape and pulse power will be mandatory and is feasible in a seeded High Gain Harmonic Generation (HGHG) approach. Freely selectable photon polarization and wavelength tuning is essential for the new light sources as the proposed BESSY-Soft X-ray FEL user facility. Variable pulse repetition rates and alterable pulse patterns, including fast switching of the full bunch train or single bunches to different parallel operating FEL-Lines thus are foreseen for the facility. BESSY’s recently published Technical Design Report for a multi-user facility[1] is based on a cascaded HGHG-scheme[2]. The combination of a superconducting CW linac, based on the TESLA-development, and the stability and reproducibility of the HGHG-scheme together with frequency and polarization tuning using gap-variable APPLE III-type undulators are the key elements for the proposal presently under evaluation by the funding agency. The status of the BESSY HGHG-FEL project is reviewed in the paper.