Knowledge of the ion species emitted by the SPT-100 Hall thruster provides
considerable insight to basic thruster performance. To gain detailed inform
ation concerning the composition of the plume plasma, a unique molecular be
am mass spectrometer was constructed to interrogate the exhaust plume, An i
nvestigation using this technique yielded both the ionization fraction of t
he xenon propellant and a qualitative analysis of the minority plasma speci
es 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 fa
cility gasses including nitrogen, oxygen, and water vapor was documented, a
s well as trace carbon ions caused by sputtering of graphite surfaces in pr
oximity of the thruster.