Date of Award

4-2025

Degree Name

Doctor of Philosophy

Department

Mechanical and Aerospace Engineering

First Advisor

Kristina Lemmer, Ph.D.

Second Advisor

William Liou, Ph.D.

Third Advisor

Robert Lobbia, Ph.D.

Fourth Advisor

Michael McDonald, Ph.D.

Keywords

Cathode, electric propulsion, plasma, turbulence

Abstract

Hall effect thrusters (HETs) are the most widely used electric propulsion (EP) device for in-space satellite propulsion. EP devices provide high efficiencies for extended mission lifetimes and low thrust for precision maneuvers. The hollow cathode is a critical HET component and undergoes extensive lifetime testing in addition to the lifetime testing of the full HET. Significant density gradients, non-Maxwellian electron behavior, and magnetic fields sufficiently strong enough to magnetize electrons in hollow cathode plumes create favorable conditions for the formation of plasma instabilities, which have an impact on cathode lifetime. Cathode qualification testing is typically conducted independently and includes a downstream cylindrical anode and a magnetic field simulator that mimics the HET environment. It is critical to operate lone cathode experiments in environments that best match the conditions the cathode experiences in the full thruster to accurately model cathode lifetime. However, it remains unclear which cathode operational variables significantly influence the dynamic and turbulent profile in the cathode plume to best replicate the thruster profile. This work shows an investigation of key variables in standalone cathode testing, including variations in magnetic field, background pressure, induced current oscillations on the anode, and cathode mass flow rate, to evaluate how independent cathode operation compares to thruster operation. High-speed diagnostics were employed to investigate the downstream plasma of the independent cathode. Fluctuations in the ion density were used to spatially map the wave energy divided into two regimes: a low-frequency band (100 Hz – 250 kHz), encompassing large-scale oscillations such as cathode breathing mode and azimuthal ion drifts; and a high-frequency band (250 kHz – 14 MHz), capturing small-scale turbulences in the plume (e.g., ion acoustic turbulence and lower hybrid drift modes). It was shown that the onset of the azimuthal ion drift varies with respect of magnetic field strength for argon, krypton, and xenon, and is dominant in the frequency spectra at HET magnetic field strengths.

Regions of heightened high-frequency wave energy were identified, including along the steep density gradient in the cathode plasma region, near the cathode exit plane away from the centerline, and near the anode entrance downstream. The wave energy along the cathode density gradient was primarily influenced by changes in mass flow rates, and relatively unaffected by changes in background pressure. In contrast, the regions near the cathode exit plane (off-centerline) and near the anode entrance were significantly affected by background pressure, mass flow rates, and induced cathode oscillations. Lowering the flow rate of independent cathode operation can enhance local turbulence for a closer match to the HET profile, while global changes to the neutral pressure by adjusting the number of cryogenic pumps might not be sufficient in replicating the dynamic and turbulent environment to the HET.

Access Setting

Dissertation-Open Access

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