PASP Plus solar array parasitic current collection flight results

Exposed positive potential surfaces of spacecraft solar cells (interconnects and cell sides) collect electrons from the surrounding plasma. In low-Earth-orbit, this current is a drain on the array power that can be significant for high-power arrays, and this current influences the floating potential of the spacecraft. One objective of the Air Force PASP Plus (Photovoltaic Array Space Power Plus Diagnostics) experiment is an improved understanding of parasitic current collection. We examined the flight data for the arrays facing the ram direction. Overall, the effective current collection area rises about two orders of magnitude as the applied bias rises one order of magnitude, as is typical when snapover plays a role. The collecting area has a weak dependence on plasma density, with larger collecting areas corresponding to lower densities (longer debye length). This dependence on plasma density is stronger for lower densities and weaker for higher densities. There is a large amount of scatter in the measurements, with the current collected under similar conditions varying by up to a factor ten. Further laboratory and flight experiments are needed to determine the cause of the scatter. Even with this ambiguity, PASP Plus results can be used to quantify this effect by spacecraft designers and have already been used to validate an analytic model of this process.