A Caltech Library Service

Plasma Surface Interactions in LaB₆ Hollow Cathodes with Internal Xe Gas Discharge


Guerrero Vela, Pedro Pablo (2019) Plasma Surface Interactions in LaB₆ Hollow Cathodes with Internal Xe Gas Discharge. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4CW7-2K35.


The ultimate goals of space vehicles are to move faster, further, and more reliably in the space environment. Electric propulsion (EP) has proven to be a necessary technology in the exploration of our solar system ever since its working principle was empirically tested in space in 1964. Thanks to the high exhaust velocities of ionized propellant gases, EP enables efficient utilization of the limited supply of propellant aboard spacecrafts. This technology has opened the possibility of long distance autonomous space missions.

EP devices require electron sources to ionize the propellant gas and to neutralize charges that are leaving the spacecraft. In modern EP thrusters, this is achieved by the use of hollow cathodes -- complex devices that employ low work function materials to emit electrons. Hollow cathodes using polycrystalline LaB6 inserts are attractive candidates for long duration EP based space missions. However, the physics behind LaB6 hollow cathode operation has not been studied in detail, which limits the possibility of their optimization. This work presents an integrated experimental and computational approach to investigate LaB6 hollow cathode thermal behaviour and the interplay between LaB6 insert surface chemistry and xenon plasma.

Our investigation of the thermal behaviour of LaB6 cathodes led to the unexpected discovery of a thermal transient when a new insert is first used. Specifically, we observed that the cathode temperature decreases by approximately 300 degrees over 50 hours before reaching steady state. This finding suggests a beneficial dynamic evolution of the cathode's chemical state when it interacts with its own plasma. This evolution is intrinsic to cathode operation and can only be precisely understood when the multiphysic nature of the cathode is self-consistently simulated. Thus, we built a numerical platform capable of combining the plasma, thermal and chemical behavior of a discharging hollow cathode. Simulations incorporating different neutralization models, inelastic ion-surface interaction and heterogeneous chemical evolution led to two major conclusions. First, simulations predicted a significant reduction of the LaB6 work function (0.42~eV) compared to previously reported baseline values, which is of paramount importance for EP thruster efficiency and operational lifetimes. Second, simulations suggested that the interaction between xenon low energy ions (< 50 eV) and the LaB6 surface occurs following a two step neutralization mechanism. The predicted work function reduction was experimentally confirmed by photoemission spectroscopy. Furthermore, using a combination of crystallographic analysis, scanning electron microscopy and profilometry, we demonstrated that work function reduction is caused by the creation of a crystallographic texture at the LaB6 surface upon interaction with Xe plasma. In addition, we postulated the existence of a work function enhancing mechanism of secondary importance, which can be explained by forced cationic termination of plasma exposed crystals.

Our results revealed the unexpected phenomenon of work function reduction upon plasma exposure of LaB6. These findings suggest that LaB6 hollow cathodes may outperform current technologies and become the component of choice in EP thrusters for future space missions.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Plasma propulsion, Electric propulsion, Plasma-surface interactions, Cathode, Hollow cathode, LaB6, lanthanum hexaboride, ion-surface interactions, cathode modelling, hall thrusters, ion engines
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Space Engineering
Minor Option:Electrical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Meiron, Daniel I.
Thesis Committee:
  • Meiron, Daniel I. (chair)
  • Shepherd, Joseph E.
  • Austin, Joanna M.
  • Polk, James E.
Defense Date:8 May 2019
Funding AgencyGrant Number
Glenn Research CenterUNSPECIFIED
Record Number:CaltechTHESIS:06032019-100503451
Persistent URL:
Related URLs:
URLURL TypeDescription ItemArticle adapted for Ch.2 adapted for Ch.3
Guerrero Vela, Pedro Pablo0000-0001-5766-2038
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11673
Deposited By: Pedro Pablo Guerrero Vela
Deposited On:04 Jun 2019 22:33
Last Modified:02 Nov 2021 19:40

Thesis Files

PDF - Final Version
See Usage Policy.


Repository Staff Only: item control page