Kärcher V, Roling S, Samoylova L, Buzmakov A, Zastrau U, Appel K, Yurkov M, Schneidmiller E, Siewert F, Zacharias H
Physikalisches Institut, Westfälische Wilhelms-Universität, 48149 Münster, Germany.
European XFEL GmbH, 22869 Schenefeld, Germany.
J Synchrotron Radiat. 2021 Jan 1;28(Pt 1):350-361. doi: 10.1107/S1600577520014563.
For the High-Energy-Density (HED) beamline at the SASE2 undulator of the European 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 the Synchrotron Radiation Workshop (SRW) software in order to investigate the impact 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 τ = 0.23 fs and deduces one of τ = 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 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.
对于欧洲X射线自由电子激光装置（European XFEL）的SASE2波荡器中的高能量密度（HED）光束线，已搭建了一个硬X射线分离与延迟单元（SDU），可实现光子能量在5 keV至24 keV之间的时间分辨泵浦/探测实验。SDU的光学布局基于几何波前分割和多层布拉格镜。在24 keV时最大延迟为Δτ = ±1 ps，在5 keV时为Δτ = ±23 ps。借助同步辐射工作室（SRW）软件进行了随时间变化的波前传播模拟，以研究光学布局对XFEL脉冲时空特性的影响，包括分束器和复合器边缘的衍射以及所有八面镜的三维形貌。辐射由代码FAST从噪声中产生，该代码模拟了自放大自发辐射（SASE）过程。对受干扰干涉图样进行快速傅里叶变换评估，对于理想镜面得出相干时间τ = 0.23 fs，对于实际镜面推导出τ = 0.21 fs，误差Δτ = 0.02 fs，该误差小于SASE2脉冲逐次波动导致的偏差。通过复合折射透镜对波前进行聚焦，以便在直径为20 µm（半高宽）的光斑宽度内实现几百mJ/mm的注量。对于相干时间内的脉冲延迟，相干效应和光学缺陷会使峰值强度增加200%至400%。此外，还讨论了HED光束线中两个偏置镜的影响。此外，我们展示了沿光轴在焦斑周围Δz = ±3 mm范围内的注量分布。模拟结果表明，SDU镜的形貌足以支持X射线泵浦/X射线探测实验。
