Metrology and characterization of two optical delay line mirrors before and after B4C coating at European XFEL

Abstract

The European XFEL generates extremely short and intense x-ray laser pulses, with high coherence and diffractionlimited divergence. It generates ultrashort x-ray flashes, 27 000 times per second, with a brilliance that is a billion times higher than that of the best conventional x-ray radiation sources. Due to these extreme beam characteristics, the x-ray mirrors to transport and focus the beam need to be coated to protect them against beam damage. The surface quality of these mirrors before the coating is in the order of 2 nm peak-to-valley and 200 km radius on average. The ultra-flat x-ray mirrors of ultimate precision are important in order to preserve the quality of the beam delivered to the experiments and the coating must not compromise the surface quality. This manuscript presents a preliminary study of the surface quality of 2 mirrors that will be part of the Optical Delay Line (ODL) before and after coating using Fizeau and White Light Interferometry.