Clustering of Hall effect thrusters for high-power electric propulsion applications

This thesis presents research aimed at understanding the technical issues related to operating multiple Hall effect thrusters in close proximity to each other. This will facilitate development of electric propulsion systems capable of operating at power levels beyond the current state of the art.; An extensive array of plume data was obtained using a variety of plasma diagnostics. Measurements were taken downstream of a cluster of four thrusters, each of which was coupled to its own hollow cathode and operated from its own set of power supplies. Comparing data obtained in this configuration to measurements taken in the plume of a single thruster showed that three of the most basic properties in the cluster plume: plasma density, electron temperature, and plasma potential, could be predicted based solely on knowledge of a single thruster. Predictions made using the methods presented in this dissertation appear to be accurate to within the margin of error of typical plasma diagnostics.; Secondary properties such as the ion current density and ion energy spectrum were also studied in the cluster plume. It was found that the beam profile of a cluster is slightly narrower than predicted by linear superposition of the contributions from each individual engine. A particle tracking algorithm revealed this behavior to be the result of low-energy ions being preferentially deflected downstream by the unique plasma potential profiles in the cluster plume. Measurements of the ion energy spectrum showed a significant increase in ions occurring at energy to charge ratios below the main peak in the distribution when multiple thrusters were operated. This appears to indicate an increase in elastic scattering due to clustering.; Finally, several alternative cluster configurations have been studied to examine parallel and shared cathode operation. It was found that parallel operation generally caused one cathode to dominate the discharge. When multiple thrusters were coupled to a single cathode, the plume properties could no longer be predicted using simple analytical formulas. The dramatic changes in plume properties observed in this configuration are the result of poor cathode coupling caused by operation with a distant cathode.