Development of multilayer coatings for solar orbiter EUV imaging telescopes

Abstract

Since more than 20 years, Laboratoire Charles Fabry and Institut d’Astrophysique Spatiale are involved in development of the EUV multilayer coating for solar imaging. Previous instruments, such as the SOHO EIT and STEREO EUVI telescopes, employed the Mo/Si multilayer coatings, which offered at that time the best efficiency and stability. We present here recent results of the development of highly efficient EUV multilayers coatings at 17.4 nm and 30.4 nm for the Solar Orbiter mission. New multilayer structures, based on a combination of three materials including aluminum, have been optimized both theoretically and experimentally. We have succeeded to reduce interfacial roughness of Albased multilayers down to 0.5 nm via optimization of the multilayer design and the deposition process. The EUV peak reflectance of Al/Mo/SiC and Al/Mo/B₄C multilayer coatings reaches 56% at 17.4 nm, the highest value reported up to now for this wavelength. We have also optimized specific bi-periodic structures that possess two reflection bands in the EUV range with high spectral selectivity. The EUV reflectivity of these Al-based dual-band coatings are compared with the Si/Mo/B₄C baseline coating for Solar Orbiter. Since the stability of reflecting multilayer coating is an important issue for space missions, we have also studied the temporal stability as well as the resistivity of the coatings to thermal cycling and to proton irradiation. Experimental results confirm that Al/Mo/SiC and Al/Mo/B₄C multilayer coatings are good candidates for the Solar Orbiter EUV imaging telescopes.