Propellant lonization and mass spectral measurements in the plume of an SPT-100
Knowledge of the ion species emitted by the SPT-100 Hall thruster provides considerable insight to the basic thruster performance. In order to gain detailed information concerning the composition of the plume plasma a custom molecular beam mass spectrometer was constructed to interrogate the exhaust plume. An investigation using this technique yielded both the ionization fraction of the xenon propellant as well as a qualitative analysis of the minority plasma species attributed to ground-test facility interactions. The plasma was found to consist of 89% Xe+, 11% Xe2+, and 0.2% Xe3+. The existence of parasitic facility gasses including nitrogen, oxygen and water vapor was documented, as well as trace carbon ions caused by sputtering of graphite surfaces in proximity of the thruster. Through a species-dependent analysis of the ion energy distribution the products of momentum transfer collisions between plume ions were uncovered. These products appeared as highenergy tails on the ion voltage distribution function representing ions at voltages greater than that applied to the thruster discharge.
