Siewert F, Buchheim J, Gwalt G, Bean R, Mancuso A P
Helmholtz Zentrum Berlin für Materialien und Energie, Department Optics and Beamlines, Albert-Einstein-Str. 15, 12489 Berlin, Germany.
European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany.
Rev Sci Instrum. 2019 Feb;90(2):021713. doi: 10.1063/1.5065473.
Recently, the European X-Ray Free Electron Laser (XFEL) has successfully produced its first X-ray photon pulse trains. This unique photon source will provide up to 27 000 photon pulses per second for experiments in different fields of science. In order to accomplish this, ultra-precise mirrors of dedicated shape are used to guide and focus these photons along beamlines of up to 930 m in length from the source in the undulator section to the desired focal point at an experimental station. We will report on a Kirkpatrick-Baez-mirror pair designed to focus hard-X-rays in the energy range from 3 to 16 keV to a 100 nm scale at the SPB/SFX instrument of the European XFEL. Both mirrors are elliptical cylinder-like shaped. The figure error of these 1 m long mirrors was specified to be better than 2 nm pv in terms of the height domain; this corresponds to a slope error of about 50 nrad rms (at least a best effort finishing is requested). This is essential to provide optimal experimental conditions including preservation of brilliance and wavefront. Such large and precise optics represents a challenge for the required deterministic surface polishing technology, elastic emission machining in this case, as well as for the metrology mandatory to enable a precise characterization of the topography on the mirror aperture. Besides the slope errors, the ellipse parameters are also of particular interest. The mirrors were under inspection by means of slope measuring deflectometry at the BESSY-NOM slope measuring profiler at the Helmholtz Zentrum Berlin. The NOM measurements have shown a slope error of 100 nrad rms on a aperture length of 950 mm corresponding to a residual figure deviation ≤20 nm pv for both mirrors. Additionally we found a strong impact of the mirror support conditions on the mirror shape finally measured. We will report on the measurement concept to characterize such mirrors as well as to discuss the achieved results.
最近，欧洲X射线自由电子激光装置（XFEL）成功产生了首批X射线光子脉冲序列。这种独特的光子源每秒将为不同科学领域的实验提供多达27000个光子脉冲。为实现这一点，使用了具有特定形状的超精密镜面，以将这些光子沿着长达930米的光束线从波荡器段的源引导并聚焦到实验站的所需焦点。我们将报告一对柯克帕特里克-贝兹镜，其设计用于在欧洲XFEL的SPB/SFX仪器中将能量范围为3至16keV的硬X射线聚焦到100纳米尺度。两面镜子均为椭圆圆柱状。就高度域而言，这些1米长镜子的面形误差规定优于2纳米峰谷值；这对应于约50纳弧度均方根斜率误差（至少要求尽最大努力完成）。这对于提供包括保持亮度和波前在内的最佳实验条件至关重要。如此大型且精密的光学元件对所需的确定性表面抛光技术（在这种情况下是弹性发射加工）以及对镜面孔径形貌进行精确表征所必需的计量学而言都是一项挑战。除了斜率误差外，椭圆参数也特别受关注。这些镜子在柏林亥姆霍兹中心的BESSY-NOM斜率测量轮廓仪上通过斜率测量偏折术进行检测。NOM测量表明，在950毫米的孔径长度上，两面镜子的斜率误差均方根为100纳弧度，对应残余面形偏差≤20纳米峰谷值。此外，我们发现镜子的支撑条件对最终测量的镜子形状有很大影响。我们将报告用于表征此类镜子的测量概念，并讨论所取得的结果。
