Investigation of Low Discharge Voltage Hall Thruster Characteristics and Evaluation of Loss Mechanisms.

Abstract

During the early development stages of Hall thruster technology, plasma research and propulsion advancements centered primarily on 300-V, 1600-s specific impulse operation. Since the first Hall thruster firing on a Soviet satellite in 1972, extensive investigations of the plasmadynamic discharge phenomena and operating characteristics progressed the propulsion concept to a high level of performance suitable for a wide range of near-earth maneuvers and interplanetary missions. The expanded performance envelope is primarily a function of improvements in thruster lifetime, thermal margin, discharge stability, and power system capability. Advancements in Hall thruster propulsion systems enable a wider range of input parameters to the thruster, including the applied anode-to-cathode potential. Operation in the low discharge voltage regime is associated with a decline in total thruster efficiency. This dissertation is intended to investigate low-voltage Hall thruster physics, identify dominant performance loss mechanisms, and determine the discharge characteristics that drive efficiency.

A systematic, experimental investigation of low-voltage Hall thruster performance and plume properties led to the conclusion that reduced electron temperature in the discharge was correlated with diminished Joule heating losses and a lower ionization cost per beam ion. However, the reduced electron temperature also decreased the ionization rate coefficient, and corresponded to an escalation of electron current to the anode to sustain ionization processes. In addition, divergence of the kinetic ion jet limited the component of axially directed thrust and reduced the total thruster efficiency. Two jet-mode Hall thruster operating regimes were discovered for low-voltage operation, corresponding to ionization instabilities in the discharge and additional electron current to the anode. These modes are methodically characterized and potential causes are hypothesized.

During the course of this research, corollary studies on Faraday probe design, facility effects, and data analysis techniques improved accuracy of current density profiles and far-field plume properties. Faraday probe uncertainty is difficult to quantify, and therefore is often employed for qualitative analysis of electric propulsion plumes. The reduction in Faraday probe measurement uncertainty and the increased capability to approximate on-orbit plume expansion are significant improvements for comparison with numerical simulations and analysis of thruster performance.