Metrology measurements of mounted mirrors for EuXFEL HED Split and Delay Unit

For the High Energy Density Instrument (HED) at the European XFEL, a split and delay unit (SDU) is built. This unit covers photon energies in the range between 5 keV and 24 keV and enables time-resolved x-ray pump / x-ray probe experiments as well as sequential diffractive imaging on a femtosecond to picosecond time scale [1][2]. The SDU consists of 7 plane mirrors, 190 mm long with different sets of reflective multilayer coatings and one 120 mm long beam splitter mirror. Multilayer mirrors are used to achieve significant reflectivity and facilitate incident angles large enough that correspond to maximum possible delays in the picosecond range. The x- ray FEL pulses are split at a sharp edge of a wavefront beam-splitter (BS) and then propagated through two delay branches (upper delay path: U1, U2 and lower delay path: D1, D2) with adjustable optical path lengths. The lower delay beam is reflected by the recombination mirror (RC) and the last mirror (S8) reflects both beams into the original direction.