Multilayer coating analysis for heat rejection on the Slit Assembly of Solar-C EUVST

Solar-C EUVST is a collaboration among Japan, US and several European countries. The goal of EUVST is to study the solar atmosphere in the extreme UV, using a spectrometer and a Slit-Jaw Imager as the main instrumentation. One of the key components of both instruments is the Slit Assembly, which is provided by the Italian Space Agency. In the baseline design, the Slit Assembly consists of a thin silicon wafer with several slits placed on the telescope focal plane. Since the slit substrate is illuminated by the focused broadband solar spectrum, the resulting thermal load may potentially be a problem. In order to maximize reflection and minimize heat absorption, and also to have high reflectance in the operating wavelength of the Slit-Jaw Imager, a specifically designed optical coating is required for the slit substrate. Due to the narrow width of the scientific slits, it is also important for the optical coating to have a thickness much smaller than the slit width, in order to avoid problem of shrinkages or partial obstruction of the slits during the deposition process. Considering these constraints, different typologies of optical coatings were analyzed in this theoretical study. We analyzed several configurations of metal and protective coatings, using a genetic algorithm to optimize the number and thickness of the layers. Among the analyzed configurations, two multilayers exhibit the best performance: a standard multilayer based on six layers of Al₂O₃ and SiO₂ and a more exotic multilayer coating based on seven layers of ZrO₂ and SiO₂.