Design of a compact hard x-ray split-delay system based on variable-gap channelcut crystals

Free Electron Lasers provide unique opportunities in investigating ultrafast atomic-scale dynamics using X-ray Pump X-ray Probe and two-pulse X-ray Speckle Visibility Spectroscopy techniques. However the performance of these techniques critically relies on maintaining the spatial overlap between the two beams at the sample location while adjusting the temporal separation between the two pulses. This presents major challenges for realizing hard X-ray split-delay optics, i.e an optical arrangement capable of generating two X-ray pulses separated in time from a single X-ray pulse. More specifically, it translates into nanoradian-scale angular precision requirements. We present here a hard X-ray split-delay optics concept using 4 channelcut crystals, where the beam path length difference can be adjusted by the linear translation of two crystals having non-parallel gaps. Having two Bragg reflections from the same monolithic crystal in the opposite direction cancels positioning errors from underlying stages. As a result, this concept has lower motion precision requirements on the hardware, and yet leads to significantly higher output beam angular stability. We present a case study for this concept that covers an energy range from 7 to 12 keV and provides a delay range from -3 to 10 ps at 9 keV.