Abstract
For the High-Energy-Density (HED) beamline at the SASE2 undulator of theEuropean XFEL, a hard X-ray split-and-delay unit (SDU) has been built enabling time-resolved pump/probe experiments with photon energies between 5 keV and 24 keV. The optical layout of the SDU is based on geometrical wavefront splitting and multilayer Bragg mirrors. Maximum delays between Δτ= ±1 ps at 24 keV and Δτ = ±23 ps at 5 keV will be possible. Time-dependent wavefront propagation simulations were performed by means of theSynchrotron Radiation Workshop (SRW) software in order to investigate theimpact of the optical layout, including diffraction on the beam splitter and recombiner edges and the three-dimensional topography of all eight mirrors, on the spatio-temporal properties of the XFEL pulses. The radiation is generated from noise by the code FAST which simulates the self-amplified spontaneous emission (SASE) process. A fast Fourier transformation evaluation of the disturbed interference pattern yields for ideal mirror surfaces a coherence time of τc = 0.23 fs and deduces one of τc = 0.21 fs for the real mirrors, thus with an error of Δτ = 0.02 fs which is smaller than the deviation resulting from shot-to-shot fluctuations of SASE2 pulses. The wavefronts are focused by means of compound refractive lenses in order to achieve fluences of a few hundred mJ mm-2 within a spot width of 20 µm (FWHM) diameter. Coherence effects and optics imperfections increase the peak intensity between 200 and 400% for pulse delays within the coherence time. Additionally, the influence of two off-set mirrors in the HED beamline are discussed. Further, we show the fluence distribution for Δz = ±3 mm around the focal spot along the optical axis. The simulations show that the topographies of the mirrors of the SDU are good enough to support X-ray pump/X-ray probe experiments.
Highlights
In the past decade, the investigation of ultrafast processes on the spatial nanoscale and at angstrom wavelengths has gained increasing interest, for which a suitable light source with well defined temporal and spatial properties is required
Dynamic processes initiated by inner-shell electrons can be studied at Freeelectron lasers (FELs) with an accelerator-based two-bunch mode (Hara et al, 2013; Lutman et al, 2013; Marinelli et al, 2015), or by means of a split-and-delay unit (SDU) (Roseker et al, 2009; Sorgenfrei et al, 2010; Sobierajski et al, 2013; Wostmann et al, 2013; Hilbert et al, 2014; Roling et al, 2014a) which may yield elemental specificity to both X-ray pump and the X-ray probe
In order to investigate the spatio-temporal properties of XFEL pulses by means of numerical simulations, two steps have to be taken into account
Summary
The investigation of ultrafast processes on the spatial nanoscale and at angstrom wavelengths has gained increasing interest, for which a suitable light source with well defined temporal and spatial properties is required. Dynamic processes initiated by inner-shell electrons can be studied at FELs with an accelerator-based two-bunch mode (Hara et al, 2013; Lutman et al, 2013; Marinelli et al, 2015), or by means of a split-and-delay unit (SDU) (Roseker et al, 2009; Sorgenfrei et al, 2010; Sobierajski et al, 2013; Wostmann et al, 2013; Hilbert et al, 2014; Roling et al, 2014a) which may yield elemental specificity to both X-ray pump and the X-ray probe In the latter case, the pulse will be separated into two partial beams which can be delayed with respect to each other by a variation of their path length. This SDU provides tunable two-color multilayer Bragg mirrors (Roling et al, 2014a) and will be installed in the year 2021 at the
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