The Darmstadt: FEL using the superconducting cw accelerator S-DALINAC

The free electron laser (FEL) at Darmstadt driven by the superconducting (sc) electron accelerator S-DALINAC performed its first lasing in December 1996. Since then it operated at wavelengths between 6.6 and 7.8 pm corresponding to electron beam energies between 29.6 and 31.5 MeV in two run time periods. By varying the electron beam energy and the gap width of the 80 period hybrid undulator the photon wavelength could be tuned continuously through the range mentioned above, which corresponds to the acceptance of the two dielectric mirrors, forming the 15 m long optical cavity. The potential of sc accelerator technology in providing high quality electron beams had to be fully exploited to ensure sufficient amplification for saturated laser operation despite of the limited peak current (2.7 A) from the S-DALINAC. Therefore a full set of beam diagnostics using transition radiation had to be developed to measure and to maintain all relevant beam parameters like pulse length, energy spread and transverse emittance. However the application of sc accelerator technology enables the special feature of this project, in particular the widely variable macro pulse structure ranging from a few microseconds to true cw operation. Historically the Darmstadt FEL is after Stanford the second installation using a sc driver accelerator and the first FEL providing a cw photon beam.