Multilayer coatings for a multichannel Wolter-like x-ray imager with large field of view and high resolution: Experimental development and study of the instrument performance

The Toroidal x-ray Imager (TXI) is a multi-channel x-ray imager with a pseudo-Wolter configuration, allowing a large field of view and a high resolution for plasma characterization. The imager has three energy bands centered at 8.7, 13, and 17.5 keV. We propose an innovative solution to fulfill the scientific requirements of TXI by combining two aperiodic multilayer coatings with different bandwidths, a narrowband on one mirror and a broadband on the other mirror for each channel. This article presents the experimental development of the W/SiC multilayer coatings on the toroidal mirrors of TXI. The coatings were deposited by the magnetron sputtering technique and characterized at-wavelength in the Physikalisch-Technische Bundesanstalt laboratory at BESSY II and at the ESRF synchrotron. The measurements indicate a good agreement with the simulations using a two-layer model for the W/SiC multilayers. Interfacial defects were modeled using roughness values of 0.3-0.4 nm at each interface. We also report on the final performance of the instrument, which was estimated using a ray-tracing code including a simulation of the multilayers based on the fit of experimental mirror reflectance spectra. Finally, we discuss the effect of the field of view on each criterion: the total throughput, the spectral purity, and the spatial uniformity in the detector plane for each channel of TXI.