Compact spectrometer for on-line photon diagnostics at FLASH

We present the design and characterization of a compact and portable spectrometer that has been realized to analyze in real time the high-order harmonic contents of the free-electron-laser beam at FLASH in Hamburg. The spectrometer can be installed at the end of any of the broad-band FEL beamlines at FLASH, to monitor in the single-shot operation the emissions of the fundamental FEL and the high-order harmonic content. The design is compact in order to obtain a portable instrument within a total envelope of less than one meter. It is based on the use of two flat-field grazing-incidence gratings and a EUV-enhanced CCD detector to cover the spectral range 1.7-40 nm (720-30 eV). The absolute response of the spectrometer, i.e. grating and detector efficiency, has been measured in the whole spectral region of operation. This allows to make calibrated measurements of the photon flux. Furthermore, the use of a bidimensional detector allows to measure also the angular divergence of the FEL beam in the direction parallel to the entrance slit. We present some experimental data of the FEL emissions taken at the beamline BL1 at FLASH. The high-order harmonic emissions have been characterized in terms of photon flux, temporal fluctuations and angular divergence. Measurements of the harmonics up to the 5th order at 3.8 nm have been done with the fundamental tuned at 19 nm. Measurements of the harmonics up to the 3rd order at 2.3 nm have been done with the fundamental tuned at 6.8 nm.