Coherent Thomson Scattering to Investigate Lower-Hybrid Instabilities in the Front Pole Region of a Magnetically Shielded Hall Thruster

This proposal plans to investigate the mechanisms behind pole erosion in Hall thrusters. Hall thrusters are an efficient alternative to chemical propulsion for in-space missions. However, to make Hall thrusters an even more competitive alternative, their lifetime must improve. Magnetically shielded Hall thrusters have the longest lifetime among Hall thrusters, but magnetically shielded Hall thrusters are susceptible to erosion of the magnetic poles. However, current simulations are not able to predict the observed levels of pole erosion. The challenge in predicting pole erosion is that the ions that erode the poles are anomalously heated before striking the poles. It is believed that the ions are heated by plasma instabilities near the poles, specifically by lower-hybrid instabilities. I plan to experimentally verify the presence of these instabilities and to improve our understanding of these instabilities. Because of the millimeter length scale of these instabilities, typical plasma diagnostics are not capable of detecting them. I propose to develop a coherent Thomson scattering diagnostic, a type of laser plasma diagnostic, to detect and characterize lower-hybrid instabilities near the poles of a magnetically shielded Hall thruster. While instabilities can often be simplified to propagate in only one or two dimensions, I plan to use coherent Thomson scattering to characterize the three-dimensional structure of these instabilities. With the measurements I provide, kinetic simulations will be able to produce an accurate model of the ion heating due to the lower-hybrid instabilities near the magnetic poles. This could potentially result in accurate pole erosion simulations, which would then be used to optimize magnetically shielded Hall thrusters to minimize pole erosion. As a result, the proposed research can have a direct impact on increasing the lifetime of Hall thrusters. This would allow Hall thrusters to support a wider range of NASA missions, including future human missions to Mars.

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