Optical design and time-dependent wavefront propagation simulation for a hard x-ray split-and-delay unit for the european XFEL

For the European XFEL [1] an x-ray split-and-delay unit (SDU) is built covering photon energies from 5 keV up to 20 keV [2]. This SDU will enable time-resolved xray pump / x-ray probe experiments as well as sequential diffractive imaging [3] on a femtosecond to picosecond time scale. The set-up is based on wavefront splitting, which has successfully been implemented at an autocorrelator at FLASH [4]. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B4C and W/B4C multilayers. Both partial beams will then pass variable delay lines. For different wavelengths the angle of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. Hence, maximum delays between +/- 2.5 ps at hv = 20 keV and up to +/- 23 ps at hv = 5 keV will be possible. The timedependent wave-optics simulations have been done with SRW software, for the fundamental at h = 5 keV. The XFEL radiation was simulated using an output of timedependent SASE code FAST. Main features of the optical layout, including diffraction on the splitter edge, and optics imperfections were taken into account. Impact of these effects on the possibility to characterize spatialtemporal properties of FEL pulses are analyzed.