A hard x-ray split-and-delay unit for the HED Instrument at the European XFEL Sebastian Roling 1* , Victor Kärcher 1 , Liubov Samoylova 2 , Karen Appel 2 , Stefan Braun 3 , Peter Gawlitza 3 , Frank Siewert 4 , Ulf Zastrau 2 , Matthias Rollnik 1 , Frank Wahlert 1 , and Helmut Zacharias 1* 1 Physikalisches Institut, WWU Münster, Wilhelm-Klemm Straße 10, 48149 Münster, Germany 2 European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Germany 3 Fraunhofer Institut IWS, Winterbergstraße 28, 01277 Dresden, Germany 4 Helmholtz-Zentrum für Materialien und Energie, Albert-Einstein-Straße 15, 12489 Berlin, Germany *corresponding authors: s_roli02@uni-muenster.de, h.zacharias@uni-muenster.de ABSTRACT For the High Energy Density Instrument (HED) at the European XFEL a hard x-ray split-and-delay unit (SDU) is built covering photon energies in the range between 5 keV and 24 keV. This SDU enables time-resolved x-ray pump / x-ray probe experiments as well as sequential diffractive imaging on a femtosecond to picosecond time scale. The set-up is based on wavefront splitting that has successfully been implemented at an autocorrelator at FLASH. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B 4 C and W/B 4 C multilayers. Both partial beams then pass variable delay lines. For different photon energies the angle of incidence onto the multilayer mirrors is adjusted in order to match the Bragg condition. Hence, maximum delays between +/- 1 ps at 24 keV and up to +/- 23 ps at 5 keV will be possible. Time-dependent wave-optics simulations are performed with Synchrotron Radiation Workshop (SRW) software. The XFEL radiation is simulated using the output of the time-dependent SASE code FAST. For the simulations diffraction on the edge of the beam-splitter as well as height and slope errors of all eight mirror surfaces are taken into account. The impact of these effects on the ability to focus the beam by means of compound refractive lenses (CRL) is analyzed. 1. INTRODUCTION In this paper a novel x-ray split-and-delay unit based on wavefront beam splitting and multilayer mirror coatings is described. It covers photon energies between h 5 keV and h 24 keV. With this spectral range the SDU will enable jitter-free hard x-ray pump/ hard x-ray probe experiments at the High Energy Density Instrument (HED) [1] at the SASE 2 undulator of the European XFEL [2]. In the hard x-ray regime single coatings at grazing angles larger than  = 0.2° show a high absorbance and a small reflectivity. Thus, multilayer mirrors are utilized in order to facilitate sufficiently large incident angles corresponding to maximum possible delays in the picosecond range. The x-ray pulses are split at a sharp edge of a wavefront beam-splitter and then propagate through two delay branches with adjustable optical path lengths. For the XUV and soft x-ray spectral regime a similar set-up with carbon coatings instead of multilayer mirrors has been integrated into the FLASH SASE FEL [3]. With this device the spatio-temporal coherence properties [4,5] as well as the pulse duration [6] of a soft x-ray FEL have successfully been measured for the first time. Further, ionization dynamics in expanding clusters [7] and in iodine molecules [8] have been investigated by XUV pump / XUV probe spectroscopy, the ultrafast heating of hydrogen has been studied [9,10], and femtosecond sequential imaging has been realized for the first time [11]. The new SDU at the HED Instrument at the European XFEL will enable similar experiments in the hard x-ray spectral regime. In order to evaluate the influence of the device on experiments with focused hard x-ray pulses, time-dependent wave- optics simulations have been performed by means of Synchrotron Radiation Workshop (SRW) software for SASE