Comparison of Discharge Plasma Parameters in a 30-cm NSTAR Type Ion Engine with and without Beam Extraction

Double Langmuir probe measurements are presented over a two-dimensional array of locations in the near Discharge Cathode Assembly (DCA) region of a 30-cm diameter ring cusp ion thruster. Discharge plasma data taken with and without beam extraction exhibit a broadening of the high on-axis electron temperature region when a beam is extracted. Electron number density and temperature contours are presented for similar discharge currents and voltages, but different cathode and main mass flow rates. Decreasing the discharge cathode flow rate tends to broaden the high on-axis electron temperature region possibly resulting from transition of the discharge cathode from spot to plume mode. Electron temperature magnitudes, 2 – 7 eV, are comparable to those measured by other researchers in electron bombardment discharge plasmas. Number density contours, with a maximum of approximately 2x10 cm on centerline, show very little variation over the range of operating conditions investigated. Number densities are similar to data taken by other researchers and are high enough to account for the measured erosion of the DCA in the 8200 hour wear test, as calculated by Williams, due to singly ionized xenon accelerating away from a potential hill back towards the DCA.