Discharging spacecraft through neutral gas release: Experiment and theory

We describe experimental and theoretical research related to the mitigation of spacecraft charging by the release of neutral gas through specially des igned nozzles. The experiments were conducted in the large Space Physics Si mulation Chamber (SPSC) at the Naval Research Laboratory. A realistic near- Earth space environment can be simulated in this device for which minimum s caling needs to be performed to relate the data to space plasma regimes. Th e environment of the SPSC is similar to that encountered by spacecraft ill low Earth orbit. The experimental arrangement consists of an aluminum cylin der which can be biased to high voltage (0.4 < V < 10 kV). The cylinder inc orporates a neutral gas release valve designed for millisecond release time s, a pressure-regulated neutral gas reservoir, and variable Mach number noz zles. After the cylinder is charged to high voltage the neutral gas is rele ased, inducing a breakdown of the gas in the strong electric field about th e cylinder. Collection of ions from the newly created dense plasma, along w ith secondary electron emission from the cylinder surface, provides the ret urn current necessary for grounding the body. We treat the breakdown theore tically as a Townsend discharge and use the fundamental assumption of expon ential electron growth as one proceeds from the cathode toward the anode du ring neutral gas breakdown. In addition, the nozzle release of neutral gas is modeled, and a simple linear spatial dependence of the applied potential is assumed. This basic model produces quite good results when compared to the experiment.