Modeling of EUV multilayers and filters for solar physics and evaluation of tabulated optical constants
Résumé
Solar observation missions are essential as they provide data to enrich predictive models about flares, coronal mass ejections (CMEs), solar wind acceleration and heating mechanisms of the solar corona. Such coronal events are characterized by energies corresponding to the emission of extreme ultraviolet (EUV) rays, making them the most relevant to image and analyze [1], [2].
Multilayer mirrors for the EUV (wavelengths from 10 to 100 nm) enabled the imaging and analysis of the solar corona and related phenomena. Since the first EUV telescope aboard a satellite (SoHO mission, 1995 [3]) using Mo/Si periodic multilayers, new interference coatings with higher efficiency have been developed with more than two different materials repeated periodically or aperiodically. Advanced coatings made of three-material multilayers can be found in the EUV telescope of the Solar Orbiter satellite (to be launched in 2020) [4], [5].
More complex multilayers and filters with higher efficiencies require an increased knowledge of EUV optical constants for materials, to carry out optics simulations. The determination of material optical constants in the EUV range is not straightforward and different optical constant values can be found in the literature. Measurement methods of optical constants are sensitive to material absorption edges in this region, as well as surface and interface defects like roughness, diffusion and oxidation. As a result, uncertainties in optical constant values may induce unreliable simulations and by extension, compromise the experimental performance of the final coatings.
In the following work, we are bringing into focus several discrepancies in optical constant values between 10 nm and 80 nm wavelengths, as well as their consequences in simulations. Investigated materials include Al, B4C, Mg, Mo, SiC and Zr. Experimental spectra are compared using the IMD software, including transmittance data of two filters and reflectivity data of tri-material multilayers developed for the Solar Orbiter EUV telescope [6]. Surprisingly, we determine that there are several important materials / energy ranges where the optical constants are still poorly known. These on-going analyses lead us to select the most accurate optical constants values in the EUV range for a few interesting materials. Such results are promising for the development of a new generation of high- efficiency instruments for heliophysics.