UV Exposure Experiments for 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 (Rs) 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 Rs 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 UVexposure 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 overglass-interconnectcell (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.