Date of Award

8-2020

Degree Name

Doctor of Philosophy

Department

Mechanical and Aerospace Engineering

First Advisor

Dr. Kristina Lemmer

Second Advisor

Dr. HoSung Lee

Third Advisor

Dr. Robert Lobbia

Fourth Advisor

Dr. Peter Peterson

Keywords

Hall thruster, electric propulsion, plasma, plasma diagnostics

Abstract

Magnetically shielded Hall effect thrusters (MSHETs) are a variant of Hall effect thrusters (HET) that provide the favorable thrust-to-power ratio and high specific impulse of traditional stationary plasma type (STP) HETs but with an extremely long lifetime. MSHETs exhibit unique discharge plasma oscillations compared to traditional unshielded HETs. In this study, measurements of plasma oscillations in the state-of-the-art 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) were obtained at various discharge voltages while holding other operation parameters constant. Data were collected using high-speed imaging, voltage, current, and plasma probes. The thruster exhibits two dominant oscillation modes within the discharge channel. The first mode is characterized as broadband turbulent oscillation in discharge current, and the second mode is a sinusoidal-like oscillation in discharge current. The oscillations in the discharge channel for both modes are predominantly global throughout the entire discharge and plume. The centrally mounted cathode exhibits a combination of global and localized "spoke" oscillation modes in the ExB direction. Discrete Fourier Transform (DFT) analysis of the high-speed data show the frequency and spoke number dependent dynamic coupling behaviors between the discharge channel and cathode. Moreover, the shape of the cathode mode was quantified at each operating condition using a phase surface analysis technique of the high-speed imaging data. Data from high-speed plasma probes show that oscillations in the cathode permeate into the discharge channel and could be, in part, responsible for driving oscillations in the discharge channel. A novel technique for reconstructing temporally resolved ion energy distribution functions (IEDF(t)) was developed and tested on several plasma sources. This method, called the high-speed retarding potential analyzer (HSRPA), combines a retarding potential analyzer and a high-speed transimpedance amplifier with an empirical transfer function technique previously employed in the spatiotemporal reconstruction of high-speed Langmuir probe data. The analysis technique was successful in reconstructing the IEDF(t) of a modulated gridded ion source and of a 200-W laboratory Hall thruster. The HSRPA was less effective when used to collect IEDF(t) data in the high polar angle regions of the HERMeS thruster plume, but it was able to provide ion energy dependent spectral content. The HSRPA developed was a preliminary proof-of-concept that worked successfully and with subtle improvements would provide a powerful diagnostic tool for enlightening further studies of plasma oscillations in MSHETs.

Access Setting

Dissertation-Campus Only

Restricted to Campus until

8-31-2021

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