Ph.D. Thesis Defense

Julian Lopez-Uricoechea

(Faculty Advisor: Professor Mitchell Walker)

Thursday, June 4

3:00 - 4:00 p.m.

Guggenheim 244

Abstract:

The main goal of this work was to provide a better understanding of anomalous electron momentum transport along magnetic field lines in a Hall effect thruster (HET) for the purpose of improving the predictive capabilities of simulations regarding HET lifetime. Along the way, we were also able to investigate anomalous electron heating, anomalous electron heat flux, and nonequilibrium electron phenomena. The study was carried out experimentally with a magnetically shielded HET and with incoherent laser Thomson scattering (ILTS) as the plasma diagnostic. We developed an ILTS data processing method that directly quantifies the electron nonequilibrium through the skew and excess and kurtosis of the electron velocity distribution function (EVDF). We also developed a solver to quantify anomalous electron momentum transport and anomalous electron heating with ILTS measurements in a HET. The HET was operated with a low discharge voltage to push the acceleration region outside of the HET channel, which allowed us to make a 2-D (axial-radial) map of ILTS measurements the azimuthal (EVDF) across the entire acceleration region.

Regarding the variation of the anomalous electron Hall parameter along magnetic field lines, we found that it was not constant along magnetic field lines, was not proportional to the azimuthal electron drift velocity along magnetic field lines, and could not be described by existing closure models. Regarding anomalous electron heat flux, we confirmed previous experimental and simulation findings that the electron temperature is not constant along magnetic field lines, indicating that the classical conductive electron heat flux does not apply along magnetic field lines. Regarding anomalous electron heating, we found that for the five different heat flux models used, the anomalous electron heating cannot be described by anomalous resistive heating. Regarding electron nonequilibrium, we measured significant departures from equilibrium, with both the skew and excess kurtosis of the azimuthal EVDF being nonzero in the upstream region of the acceleration region. While significant attention has been given to anomalous electron momentum transport in the attempt to develop predictive HET simulations, we have found that anomalous electron heating and electron nonequilibrium are also present in the acceleration region of a HET, indicating that they need to be accounted for in predictive HET simulations.

Committee:
Dr. Mitchell Walker (advisor), School of Aerospace Engineering
Dr. Sedina Tsikata, School of Aerospace Engineering
Dr. Wenting Sun, School of Aerospace Engineering
Dr. Wensheng Huang, NASA
Dr. Vernon Chaplin, Jet Propulsion Laboratory