Free electron lasers (FEL) for the hard- and soft X-ray regime are promoted in many projects around the world and the first VUV-FEL with wavelengths down to 20 nm is already in operation at DESY in Hamburg. They open up new domains in coherent radiation, time resolution and intensity and extend current X-ray science.
The BESSY project of a 2nd generation FEL facility proposes the implementation of an externally seeded FEL with significantly improved beam quality compared to the conventional self seeded SASE-FELs.
The BESSY design as described in the technical design report1
provides full tunability of photon energies from
24 eV to 1 keV, complete polarisation control and reproducible
pulse structures with pulse lengths in the femtosecond range.
The external seeding ensures the synchronisation to other laser
sources necessary for pump-probe experiments and provides shot-to-shot reproducible pulse properties.