On the characterization of a 1 m long, ultra-precise KB-focusing mirror pair for European XFEL by means of slope measuring deflectometry

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.