First results from the online variable line spacing grating spectrometer at FLASH

Abstract A high resolution online grating spectrometer has been set up at the free electron laser in Hamburg FLASH at DESY in Hamburg. This new photon diagnostic device allows to parasitically measure the spectral distribution of the FEL laser pulses on a single shot basis and therefore gives complementary spectral and temporal pulse information for user experiments and FEL operation. The spectrometer combines a plane mirror and two plane variable line spacing (VLS) gratings and is installed at the non-monochromized beamline branch at FLASH. In contrast to a standard grating the blaze angle is optimized for the zero-order radiation transferred to the experimental stations, leaving approximately 1–10% intensity for recording the wavelength in the spectrometer. The detector, an intensified CCD camera imaging the dispersed beam on a Ce:YAG crystal, can follow the focal plane of the gratings covering a wavelength ranging from 6 to 60 nm. The resolving power of the spectrometer is expected to range from 500 to 8000 depending on the FEL wavelength.