Grating-based pulse compressor for applications to FEL sources

We present the optical layout of a reflective grating compressor specifically designed for extreme-ultraviolet FEL sources. The working principle is based on the use of a couple of constant-line-spaced gratings used at grazing incidence and illuminated in divergent light. The two possible grating configurations, namely the on-plane and off-plane, are analyzed and compared. The Group Delay Dispersion (GDD) introduced by the compressor is analytically analyzed and quantified. The spatial chirp also is considered, and its effect analyzed. The deviation from the ideal case in which the instrument is feed with a collimated beam is considered. The effect of the beam divergence on the compressor response is quantified and the attenuation of this effect by a "de-tuning" of the compressor is proposed. This solution avoids the use of a pre-collimating optics, therefore incrementing the total instrumental throughput. Finally, it is shown the optical design of an actual compressor for the FERMI FEL, that can be inserted in the optical path without any deviation or translation of the photon beam with respect to the nominal path.