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 designed nozzles. The experiments were conducted in the large Space Physics Simulation Chamber (SPSC) at the Naval Research Laboratory. A realistic near‐Earth space environment can be simulated in this device for which minimum scaling needs to be performed to relate the data to space plasma regimes. The environment of the SPSC is similar to that encountered by spacecraft in low Earth orbit. The experimental arrangement consists of an aluminum cylinder which can be biased to high voltage (0.4 < V < 10 kV). The cylinder incorporates a neutral gas release valve designed for millisecond release times, a pressure‐regulated neutral gas reservoir, and variable Mach number nozzles. After the cylinder is charged to high voltage the neutral gas is released, inducing a breakdown of the gas in the strong electric field about the cylinder. Collection of ions from the newly created dense plasma, along with secondary electron emission from the cylinder surface, provides the return current necessary for grounding the body. We treat the breakdown theoretically as a Townsend discharge and use the fundamental assumption of exponential electron growth as one proceeds from the cathode toward the anode during 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.