The BESSY soft X-ray FEL

The user requests for an optimized second-generation FEL facility in the VUV to soft X-ray range demand for reproducible ultra-short photon pulses at an energy level of milliJoule per pulse. Tunable wavelength and variable beam polarization as well as synchronization to external lasers are essential in future time-resolved pump-probe experiments. These features can be met best in a seeded HGHG approach. Free selectable pulse repetition rates and pulse pattern in combination with more than one FEL-line operating in parallel are features achievable with an RF photoinjector in combination with a CW superconducting Linac. Following these ideas, the technical design for a soft X-ray FEL user facility for the VUV to soft X-ray range, i.e. 51-1.24 nm photon wavelength is presented. The Technical Design Report for the BESSY Soft X-ray FEL based on a multi-stage High-Gain-Harmonic-Generation (HGHG) scheme was issued recently (The BESSY Soft X-ray Free Electron Laser, TDR BESSY March 2004, BESSY, Berlin).