Thermal analysis experiment to evaluate the stability of multilayer coatings in a space environment close to the sun

The next SOLO (SOLar Orbiter) mission will carry onboard the METIS (Multi Element Telescope for Imaging and Spectroscopy) instrument which will perform broad-band and polarized imaging of the visible K-corona and narrow-band imaging of the UV (HI Ly α, 121.6 nm) and EUV (He II Ly α, 30.4 nm) corona as well as in the visible spectral range. Several multilayer optics with high reflectivity in the all ranges of interest have been studied. Since SOLO will fly at the short distance from the Sun of 0.23 AU at its perihelion, a careful determination of the heat load and the solar wind effect on the multilayers must be carried in order to check if degradation occurs. To test thermal stability, a thermal analysis experiment has been conceived: the proposed multilayer structures, which are based on different pairs of materials and different capping layers design, must be subjected both to heating and cooling, reproducing the temperatures experienced in orbit. Reflectance in the EUV range of interest has been measured before and after each treatment to verify possible degradation.