On-line spectrometer for FEL radiation at FERMI@Elettra

A key diagnostic task in a free-electron laser (FEL) facility is the energy spectrum acquisition, both during the development and characterization of the source, and for the definition of the experimental conditions for the users that have to know it at each FEL shot. Since FERMI@Elettra is operated at low repetition rate (10–50 Hz), the spectrum can be easily obtained for each pulse. The design of a shot-to-shot online EUV and soft X-ray spectrometer for the FERMI@Elettra FEL facility is presented. The instrument is a grazing-incidence flat-field spectrometer equipped with a variable-line-spaced plane grating giving a constant deviation of the FEL radiation. The optical layout is detailed both to the FEL-1 (40–100 nm) and FEL-2 (10–40 nm) facilities.