Compact spectrometer for photon diagnostics of the extreme-ultraviolet free-electron-laser radiation

It is here discussed the design and the characterization of a compact spectrometer to be used at the FLASH facility (DESY, Hamburg) to acquire the spectral features of the free-electron-laser radiation in terms of high-harmonics content and shot-to-shot spectral changes. The instrument is sensitive in the 2-40 nm spectral region and very compact to be used in different beamlines. The optical design consists of an entrance slit, a spherical diffraction grating with variable groove spacing and an extreme-ultraviolet-enhanced CCD detector. The entire spectral range is covered by two interchangeable gratings installed on a rotating stage.