Student Name: Naia Butler-Craig

Advisor: Dr. Mitchell Walker

Milestone: PhD Thesis Final Examination (Defense)

Degree Program: Aerospace Engineering

Title: EXPERIMENTAL INVESTIGATION OF VARIED THRUSTER BODY BIAS ON ELECTRON BEHAVIOR IN THE NEAR-FIELD OF A MAGNETICALLY-SHIELDED HALL THRUSTER 

Abstract: Magnetically shielded Hall-effect thrusters (HETs) commonly exhibit sputter erosion of the inner front pole cover (IFPC), yet the plasma mechanisms that shape the local ion-acceleration environment at this surface remain incompletely understood. One proposed contributing factor is that the near-field electron population establishes spatially varying pressure and potential structures that modify the electric fields experienced by ions approaching the IFPC. Recent Laser Thomson Scattering (LTS) measurements have shown a pronounced radial electron pressure drop between the discharge channel and the IFPC region. Even in the presence of anomalous transport, the measured electron-pressure structure provides a direct constraint on the local electrostatic environment and on the boundary conditions that govern electron surface interaction at the IFPC. To investigate how the near-field plasma response at the IFPC depends on boundary potential, the graphite IFPC was electrically isolated and biased between –8 V and +2 V relative to ground while the thruster operated at a nominal 300 V discharge condition. Time-resolved current collection at the IFPC was recorded as a function of bias, quantifying how the magnitude and sign of net charged-particle collection vary with thruster body potential and providing an experimental link between body bias and IFPC-local plasma behavior. In the second phase, two-dimensional LTS measurements were performed at a 150 V krypton discharge condition chosen to provide sufficient signal-to-noise for Thomson scattering. For three body-bias configurations (floating, cathode-tied, and +2 V above ground), LTS provided spatially resolved electron density, temperature, and bulk radial electron drift velocity in the IFPC region. From these quantities, electron-pressure distributions, radial pressure gradients, and the convective component of the radial electron flux were determined. Because the witness-plate and LTS measurements were obtained under different operating conditions, the two datasets are not directly compared. Instead, they collectively characterize how the near-field plasma responds to changes in thruster body potential, highlighting how body bias modifies electron behavior and the electrostatic environment at the inner front pole. These findings offer insight into the plasma conditions that may influence ion impingement and erosion in magnetically shielded Hall thrusters.

Date and time: 2026-01-22, 12:30

Location: MK 317 

Committee:
Dr. Mitchell Walker (advisor), School of Aerospace Engineering
Dr. Sedina Tsikata , School of Aerospace Engineering
Dr. Wenting Sun, School of Aerospace Engineering
Dr. Kenneth Hara, Stanford University
Dr. George Williams , NASA
Dr. Mitchell Walker , School of Aerospace Engineering