The role of the electrically conductive vacuum chamber wall in the completion of the discharge circuit of a Hall effect thruster (HET) is experimentally investigated. The AerojetRocketdyne T-140 laboratory-model HET operating at a discharge voltage of 300 V, discharge current of 5.16 A, and anode flow rate of 5.80 mg/s serves as a representative HET test bed. The nominal facility operating pressure during thruster firings is 4.9 × 10 -6 Torr corrected for xenon. Two 0.91 m x 0.91 m square aluminum plates are placed adjacent to, but electrically isolated from, the walls of the stainless steel vacuum chamber at two locations with respect to the center of the thruster exit plane: 4.3 m axially downstream along thruster centerline and 2.3 m radially outward centered on the exit plane. The plates are configured in three distinct electrical configurations with corresponding measurements: a) electrically grounded plates with measurements of currents to ground, b) electrically isolated plates with measurements of floating voltages, and c) isolated but electrically connected plates with measurements of the current conducted between them. The measurements are all taken simultaneously with the discharge current oscillations of the thruster at a sampling frequency of 100 MHz. Measurements of the current conducted to ground in the electrically grounded configuration reveal that the axial and radial plates collect ion currents that are 13.6% and 10.7% of the discharge current, respectively; the collected current is coupled to the discharge current oscillations but is smaller in magnitude and phase-delayed. In the electrically connected plate configuration, 5.5% of the average discharge current is observed to flow from the axial plate to the radial plate driven by a floating voltage difference of 7.6 V; this current is uncorrelated in time with the discharge current oscillations. These results indicate that the vacuum chamber conducts current and is a recombination site for a significant number of plume ions during HET operation. NomenclatureI = current, A 2 Pb = vacuum chamber base pressure, Torr Pc = corrected vacuum chamber background pressure, Torr Pi = indicated vacuum chamber background pressure, Torr V = voltage, V Vc-g = cathode-to-ground voltage, V Vd = discharge voltage, V Vp = plasma potential with respect to ground, V Vpa = axial plate voltage with respect to ground, V Vpr = radial plate voltage with respect to ground, V ΔV = acceleration voltage, V ΔVcath = cathode coupling voltage, V

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National Lab and MacNeal-Schwendler Corp. His research includes development of innovative design methodologies and enhancement of engineering education.

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