Background pressure effects on gridded ion thruster operations

Power level requirements for near-term and future envisioned 10–100 + kW electric propulsion systems are expected to outpace the capability of ground test facilities to provide an adequate test environment that can either reasonably characterize the spaceflight performance of these thrusters or allow for known facility effect correction methods to be applicable. Limited facility pumping speed and chamber size in current vacuum test facilities are at the crux of the problem and will result in elevated background pressures and electrical coupling with the facility that have not been sufficiently investigated to be corrected for. This work focuses on the impacts of background test chamber pressure on the operation of a 30 cm gridded ion thruster that has a geometry similar to the NASA Solar Technology Application Readiness (NSTAR) thruster. The objectives of this work were to understand the measured deviations of thruster performance from observations of the NSTAR thruster in space by investigating changes in thruster operation over various background pressures. Identified in this work were observations of plume broadening in the wings of Faraday probe sweeps and high ion energies near the neutralizer and plane of the thruster. Other observations included the neutralizer transition flow rate shifted with background pressure and the need for improved correction methods for doubles-to-singles ratios. At a constant thruster throttle level, the corrected production ratio was found not to be consistent over a range of background pressures. Future high-power thruster ground testing will need to account for these observed facility effects if larger and higher pumping speed facilities are not constructed.