In a wide variety of applications such as the Hall thruster and the tokamak, understanding the plasma-material interactions which take place at the wall is important for improving performance and preventing failure due to material degradation. In the region near a surface, the plasma sheath forms and regulates the electron and ion fluxes into the material. Emission from the material has the potential to change sheath structure drastically, and must be modeled rigorously to produce accurate predictions of the fluxes into the wall. Continuum kinetic codes offer significant advantages for the modeling of sheath physics, but the complexity of emission physics makes it difficult to implement accurately. This difficulty results in major simplifications which often neglect important energy-dependent physics.
A focus of the work is on proper simulation of the sheath. The implementation of source and collision terms is discussed, alongside a brief study of the Weibel instability in the sheath demonstrating the necessity of proper collision implementation to avoid missing relevant physics.
A novel implementation of semi-empirical models for electron-impact secondary electron emission into the boundary conditions of a continuum kinetic code is presented here. The features of both high and low energy regimes of emission are represented self-consistently, and the underlying algorithms are flexible and can be easily extended to other emission mechanisms, such as ion-impact secondary electron emission. The models are applied to simulations of oxidized and clean lithium for fusion-relevant plasma regimes. Oxidized lithium has a high emission coefficent and the sheath transitions into space-charge limited and inverse modes for different parameters. The breakdown of the classical sheath results in an increase of energy fluxes to the surface, with potential ramification for applications. / Doctor of Philosophy / Great advances are being made in a variety of promising applications of plasma physics, such as the development of spacecraft thrusters and fusion devices. Many of these devices constrain the flow of plasma within a material channel, leading degradation of the wall due to particle impact to be a serious concern for durability and lifespan. The plasma sheath is a region next to these material surfaces where ions are accelerated towards the wall, while electrons are repelled. As particles from the sheath impact the material, they cause the emission of secondary particles back into the sheath. This can drastically change the expected fluxes into the material and consequently the degradation expected to occur. Continuum kinetic simulations are a valuable tool for predicting and modeling the evolution of the sheath, but they are limited in their ability to rigorously do material emission physics by their inability to directly represent particle interactions with the surface. As such, past treatments of material emission in continuum kinetics tend to sacrifice valuable energy-dependent physics for simplified models.par To facilitate better understanding of the effects of emission on the sheath and the ramifications it might have for applications, the work here seeks to develop a framework for capturing the entire range of energy-dependent emission physics within a continuum kinetic framework. The implementation relies on semi-empirical models of beam emission data, focusing on simplicity and flexibility while still capturing the separate emission mechanisms which dominate in different energy regimes.
The model is applied to simulations of lithium, an important material for fusion applications. Oxidized lithium has significantly enhanced emission properties over clean lithium, and is found to undergo a shift to non-monotonic sheath modes. The results show that the fundamental changes in the sheath structure due to the increased emission lead to greater energy fluxes into the surface.
In this work, only secondary electron emission from the impact of electrons on a surface is examined. However, the underlying algorithms are easily extended to other energy-dependent energy mechanisms, such as ion-impact secondary electron emission.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/118137 |
| Date | 23 February 2024 |
| Creators | Bradshaw, Kolter Austen |
| Contributors | Aerospace and Ocean Engineering, Srinivasan, Bhuvana, Adams, Colin, Scales, Wayne A., England, Scott Leslie |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | Creative Commons Attribution-ShareAlike 4.0 International, http://creativecommons.org/licenses/by-sa/4.0/ |
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