UV-exposure experiments for the Solar Probe Plus array

NASA´s Solar Probe Plus (SPP) will travel closer to the Sun than any previous spacecraft. During its 7-year, 24-orbit mission, SPP will make scientific measurements of the solar corona, reaching minimum perihelion at ~9.5 solar radii (R_s) from the center of the Sun. The solar array wings powering the spacecraft will operate under wide-ranging temperature and irradiance conditions, of 0 to 27×AM0 and -70 to +160°C nominally, with transient off-nominal survivability required up to 80×AM0. Over the mission duration, portions of the array may accumulate a total of over 200,000 sun hours. To properly size the array, its performance must be quantified through end-of-life, with the greatest uncertainty coming from our understanding of UV-induced degradation at high irradiance and high temperature (HIHT). The array´s short-term ability to function with no measurable degradation at the peak irradiances expected at 9.5 R_s has already been demonstrated with adequate margin on small-scale but otherwise flight-like test articles. However over 90% of the mission time will be spent by the array operating at relatively more moderate irradiances at or below 10×AM0. This paper gives an overview of results from two SPP-specific HIHT UV-exposure experiments we have performed at ~10×AM0, over durations of ~1000 hours each. The first experiment was performed in the facilities of the Boeing Radiation Effects Laboratory in Kent, WA, and it focused on the dependence of UV-induced array performance degradation on the choice of materials used in fabricating the coverglass-interconnect-cell (CIC) assemblies, at a fixed temperature of ~180°C. The second experiment was performed in the facilities of the JAXA Inner Planetary test laboratory in Sagamihara, Japan, and its focus was on measuring the temperature dependence of UV-induced array performance degradation between 120 and 240°C.