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

Citation

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. https://resolver.caltech.edu/CaltechTHESIS:06032019-100503451

Abstract

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.

Thesis Files