Grazing incidence spectrometer for the monitoring of the EUV FEL beam at DESY

A stigmatic spectrometer for the 2.5–40 nm extreme-ultraviolet (EUV) region has been realized. The design consists of a grazing-incidence spherical varied-line-spaced grating with flat-field properties and of a spherical mirror mounted in the Kirkpatrick–Baez configuration that compensates for the astigmatism. The spectrum is acquired on a fluorescent screen and intensified CCD detector that can be moved along the spectral focal curve to select the spectral region to be acquired. This stigmatic design has also spectral and spatial resolution capability for extended sources. The spectral and spatial resolution of the system has been characterized by using the emission from a hollow-cathode lamp or a laser-produced plasma. At present, the instrument is installed at the vacuum–ultraviolet free-electron-laser (VUV FEL) at DESY for the spectral monitoring of the FEL beam in the 20–40 nm region.