Metrology of micron focusing KB mirrors for SPB/SFX instrument and preliminary commissioning results at European XFEL

The European XFEL generates extremely short and intense X-ray laser pulses, with high coherence and diffraction limited divergence. Due to these outstanding characteristics of the beam, ultra-flat X-ray mirrors of ultimate precision are needed to enable a wave front preserving transport of photons from the source to the experimental hall. One of the experimental stations already in operation is the SPB/SFX (Single Particles, Clusters and Biomolecules / Serial Femtosecond Crystallography) station. This instrument is dedicated to coherent diffraction imaging of single particles and structure determination with serial femtosecond crystallography. The instrument is also designed to investigate the structure of these systems as a function of time, using the short X-ray pulses. The optics used in the photon transport tunnels as well as in the experimental station need to fulfill really challenging specifications; therefore, they need to be inspected with metrology devices and methods of comparable precision. This paper presents the metrological characterization of the micron focusing mirrors for the SPB/SFX scientific instrument done with a large aperture Fizeau interferometer. The micron focusing system consists of 2 pairs of mirrors; each pair contains one flat and one elliptical mirror. One pair is vertically reflecting and the other pair is horizontally reflecting. The central profile specification for the mirrors is less than 2 nm peak-to-valley (P-V) figure error over the main surface shape. The measurements performed in our lab are all done in grazing incidence setup and with the mirrors facing side. Analyzing these data we can account many important parameters, as the peak-to-valley of the central profile, the radius, the gravity compensation profile and the ellipse parameters. The micron focusing mirrors have been recently installed in the corresponding chamber of the SPB/SFX Optics Hutch and they will be used very soon for the next User Experiments Run.