Neutral Ingestion Effects on Plume Properties of a Radio-Frequency Plasma Discharge

To enable comparison of thruster performance across different vacuum facilities, an understanding of the effect of operating pressure on plasma plume properties generated in a helicon ion thruster architecture is required. Plasma property measurements acquired at two different vacuum facility operating pressures are used to determine the effect of neutral ingestion on the operation of a replica of the Madison Helicon Experiment at low power. The ion velocity distribution function, electron temperature, ion number density, and plasma potential are measured along the thruster axis. Plasma plume property measurements made at the high-pressure condition (3.0x10-4torr corrected for argon) are compared to values recorded at the low-pressure condition (1.2x10-5torr corrected for argon) for thruster operation at approximate to 100W radio-frequency forward power, 340G source region magnetic field strength, and 2cm3/min argon volumetric flow rate. Operation at the high-pressure condition has 31-79V lower ion energy, 0.8-6.1eV lower electron temperature, and 1.1-5.2x1014ions/m3 higher ion number densities than operation at the low-pressure condition. Changes in collision frequency and ionization balance result from increased neutral ingestion at the higher operating pressure.