Investigation of hollow cathode effects on total thruster efficiency in a 6 kW Hall thruster

Hall thrusters are being considered for interplanetary missions where improved thruster efficiency translates to an increased scientific payload mass. The objective of this thesis is to characterize the sensitivity of the total thruster efficiency to cathode location, cathode flow rate, and thruster power and to determine the thruster and cathode plume physics responsible. Experiments were conducted on a 6 kW laboratory model Hall thruster that employed a LaB6 hollow cathode. An extensive characterization of the thruster and cathode plumes was accomplished by utilizing several plasma diagnostics (Langmuir, emissive, optical, ExB, RPA, and Faraday) in the near-field and far-field plume regions. In one method, the total thruster efficiency is calculated as a function of measured thrust. A second method of calculating total efficiency is employed to compare and understand which mechanisms drive the total efficiency. In this second method, the near- and far-field plume plasma diagnostics are used to calculate the individual mass, voltage, current, and external power utilization efficiency terms and the thrust correction factor. The two methods are found to agree within 1-3%. It is shown that an internally mounted cathode improves the total efficiency by at least 2-3%, independent of power level or voltage operating conditions compared to an externally mounted cathode. Utilizing the second efficiency calculation method, the increase in total thruster efficiency for the internally mounted cathode is attributed to increased mass, voltage, and current utilization efficiencies at all operating conditions. It was also observed that for a constant discharge voltage and anode mass flow rate, the total thruster efficiency is independent of cathode flow rate for flow fractions of 5-9% of the anode mass flow rate. Lastly, a one-dimensional variable disk model was implemented to predict and characterize the plasma structure in the near-field cathode plume. The model results, which are consistent with the experimental observations, suggest that lower coupling voltages are associated with higher cathode flow rates. However, measurements show improved stability for cathode operation at the lower flow rates and higher coupling voltages. Lower flow rates reduce the required propellant mass are more attractive for any mission.