Time-dependent wave front propagation simulation of a hard x-ray split-and-delay unit: Towards a measurement of the temporal coherence properties of x-ray free electron lasers

For the European x-ray free electron laser (XFEL) a split-and-delay unit based on geometrical wavefront beam splitting and multilayer mirrors is built which covers the range of photon energies from 5 keV up to 20keV.Maximumdelaysbetween Δτ ¼� 2.5 psathν ¼ 20 keVandupto Δτ ¼� 23 psathν ¼ 5 keVwill be possible. Time-dependent wave-optics simulations have been performed by means of Synchrotron Radiation Workshop software for XFEL pulses at hν ¼ 5 keV. The XFEL radiation was simulated using results of time-dependent simulations applying the self-amplified spontaneous emission code FAST. Main featuresoftheopticallayout,includingdiffractiononthebeamsplitteredgeandopticsimperfectionsmeasured withananometeropticcomponentmeasuringmachineslopemeasuringprofiler,weretakenintoaccount.The impact of these effects on the characterization of the temporal properties of XFEL pulses is analyzed. An approachbasedonfastFouriertransformationallowsfortheevaluationofthetemporalcoherencedespitelarge wavefrontdistortionscausedbytheopticsimperfections.Inthisway,thefringesresultingfromtime-dependent two-beam interference can be filtered and evaluated yielding a coherence time of τc ¼ 0.187 fs (HWHM) for real, nonperfect mirrors, while for ideal mirrors a coherence time of τc ¼ 0.191 fs (HWHM) is expected.