The use of electrostatic probes to characterize the discharge plasma structure and identify discharge cathode erosion mechanisms in ring-cusp ion thrusters

The erosion of the discharge cathode assembly (DCA) is currently one of the lifetime limiting factors of ion thruster operation and will play an even more important role for more ambitious, future ion thruster applications requiring more throughput at higher-power. Erosion of the DCA has been observed throughout the ground-based wear testing of the 30-cm NSTAR ion thruster.; Energetic ions have been detected near the DCA, from Laser-Induced Fluorescence (LIF) measurements, that appear to be the cause of the DCA erosion, though a mechanism by which ions gain sufficient energy to sputter erode the DCA material has not been determined. This dissertation presents research aimed at characterizing the discharge chamber plasma near the DCA to determine the mechanism by which energetic ions are created and erode the DCA inside ring-cusp ion engines.; A diagnostic technique is developed to interrogate the near-DCA regions of two ion thrusters: the 30-cm FMT2 NSTAR and 40-cm LM4 NEXT engines. Both engines contain similar plasma structures. Number densities are highest along cathode centerline as the axial magnetic field near the DCA effectively confines electrons to a narrow plume. Plasma potential mappings rule out the existence of a potential-hill that has been proposed as the cause of the DCA erosion. A free standing potential gradient structure is found to form the transition between the low-potential cathode plume and the high-potential bulk discharge plasma, termed a double layer. The field-aligned double layer accelerates ions from the bulk discharge plasma towards the DCA centerline.; Measured plasma parameters and LIF velocimetry data are used to calculate an erosion rate utilizing near-threshold sputtering yield formulae. Singly-ionized xenon cannot solely account for the observed NSTAR erosion rates. Incorporation of double-ionized xenon from measured double-to-single current measurements in the plume of the 30-cm and 40-cm thrusters significantly increases the calculated erosion rates comparable to those observed in the NSTAR wear tests.; An accelerated erosion rate, similar to the Extended Life Test (ELT) erosion rate, is calculated for the case of shorting the discharge keeper to common. The additional acceleration of doubly- and singly-ionized xenon through the keeper sheath significantly increases the erosion rate by increasing the incident ion energy. The overly simplistic erosion calculation predicted a lower erosion rate for the NEXT DCA than NSTAR, due to the lower double-to-single current ratio in the plume, but wear test results do not agree pointing to the limitations of this erosion calculation and the need to improve upon it.; Methods of reducing DCA erosion include: utilizing alternative keeper materials with higher sputtering thresholds; biasing the keeper to reduce the incident ion energy; reducing the double-to-single ion current ratio; and manipulation of the double layer via the magnetic field near the DCA.