Testing of solar cells for the Solar Probe Plus mission

The Solar Probe Plus (SPP) is an upcoming mission in NASA's "Living with a Star Program" to be built by the Johns Hopkins University Applied Physics Laboratory. The spacecraft will orbit the sun for a primary mission duration of seven years, making a closest approach to the sun at a distance of 0.0442 AU. Instrumentation on SPP will focus on two primary science investigations: the sun's coronal heating and solar wind acceleration, and the production, evolution, and transport of solar energetic particles. The mission is scheduled for a launch no later than 2018. The spacecraft will have a thermal shield to protect the main portions of the spacecraft from the solar irradiance, and is powered by a pair of solar array wings. Each wing is actively cooled, and sectioned into a primary array for normal orbital operations, with a secondary for operation at high solar intensity during closest approach to the sun. Near the sun, the arrays are tilted at a high angle, withdrawing the primary array section behind the thermal shield and exposing only the secondary section, which is in the penumbra of the thermal shield to minimize the operating temperature. This secondary section will experience environmental conditions that include high solar flux, radiation, high temperature, and off-normal light incidence. It will use solar cells based on the state-of-the-art triple junction solar cells (InGaP/GaAs/Ge) for space with modifications to handle the higher irradiance and temperature. NASA Glenn Research Center tested pre-phase A solar cells for the secondary (high irradiance) section under a variety of conditions which include temperatures up to 150°C at 1 sun and 40X concentration. These measurements were made before and after irradiations that include 1 and 3.3 MeV electrons, 1, 4, and 10 MeV protons at two different fluencies. Additional testing included off-angle performance, extended temperature exposure, and UV exposure.