Metrology of MID offset mirrors before and after coating

Abstract

The Materials Imaging and Dynamics (MID) station is located at the SASE2 undulator beamline of European XFEL and has become operational in 2019. The MID instrument operates in the medium to hard X-ray range (5 - 25 keV) and its scientific focus is on time-resolved coherent X-ray scattering and diffraction studies in materials science, with particular interest in ultrafast pump-probe experiments where the pump can be either X-rays, an optical laser beam or a pulsed magnetic field. The optical setup of the MID instrument includes two vertically offset mirrors equipped with cryogenic cooling. The top mirror will be employed for grazing incidence experiments on liquid surfaces, and the bottom mirror will be used to spatially overlap two split beams generated by a “split and delay” line. The mirrors are 500 mm long and are coated with boron carbide (B4C) and platinum (Pt) in two adjacent stripes. Deterministic mirror polishing is done to compensate the gravitational sag in order to achieve a perfectly flat mirror when it is installed. The requirements were very challenging for the surface shape and the needed ion-beam deterministic polishing, so also the coating process had to be performed and monitored with particular care. We present the metrological characterization of the mirrors, carried out before and after the B4C and Pt coating, and performed with a large aperture Fizeau interferometer. The measurements were made at European XFEL’s metrology lab in grazing incidence setup and with the mirrors facing sideways. Analyzing these data, we can deduce many important parameters, as the peak-to-valley of the central profile, the bending radius, and the gravity compensation profile. We show metrological results before and after coating and give details about the calculations performed to decide whether the mirror shapes are still complying with specifications after all these processes.