Ion-energy diagnostics in the plasma exhaust plume of a hall thruster

Of primary concern with the integration of Hall thrusters oil conventional satellite designs is the possible damaging effect of high-energy exhaust io ns impinging upon spacecraft surfaces. This paper reports on measurements o f plasma ion-energy distributions within the plume of an SPT-100 Hall thrus ter using a custom-designed molecular-beam mass spectrometer. With this ins trument ion energy was measured over a complete 360-deg circumference about the thruster at a radius of 0.5 m from the exit plane and over a total inc lusive are of 260 deg at 1.0-m radius. These data uncovered the existence o f high-energy ions departing the thruster at angles exceeding 90 deg from t he thrust vector and continuing well into the backflow region of the plume. Through an analysis of the energy structure, the evidence of charge-exchan ge collisions occurring between plume ions and background neutrals was docu mented; such collisions produced anomalous distributions of ions having vol tages greater than that applied to the thruster discharge.