Characterization of a 150-mm long variable line spacing plane grating through interferometry

The European XFEL is a large facility under construction in Hamburg, Germany [1]. It will provide a transversally fully coherent X-ray radiation with outstanding characteristics: high repetition rate (up to 2700 pulses with a 0.6 milliseconds long pulse train at 10Hz), short wavelength (down to 0.05 nm), short pulse (in the femtoseconds scale) and high average brilliance (1.6x10 25 photons / s / mm 2 / mrad 2 / 0.1% bandwidth). It will have initially three main beamlines, named SASE1, SASE2 and SASE3. The last one is considered a "soft X-ray" beamline, with energies that will span from 0.25 to 3 keV, delivering photon pulses to SQS (Small Quantum System) and SCS (Spectroscopy and Coherent Scattering) experiments. The optical transport of the almost diffraction- limited beam is done using 950 mm long mirrors, cooled with InGa eutectic bath and super-polished (50 nrad RMS slope error and less than 3 nm PV residual height error). A VLS-PG (Variable Line Spacing - Plane Grating) monochromator is installed to enhance the spectral coherence of the beam. The basic characteristics for the grating substrates are: 530 mm length, InGa eutectic bath cooled and ion-beam polished with gravity sag compensation. For the initial commissioning of the beamline, a shorter grating (150 mm long) will be prepared and installed. We recently received the 150 mm long grating and we present here its characterization performed using Fizeau Interferometry. The VLS parameters are especially investigated and characterized. This grating's study can give an interesting insight in the present status of European XFEL metrology, but also additional information for the future development and characterization of the final 530 mm long grating.