Flows and instabilities in low-temperature plasmas with ionization and charge-exchange processes
Date
2022-03-30
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0002-5321-2838
Type
Thesis
Degree Level
Doctoral
Abstract
Plasma is rich with waves and instabilities, on scales ranging from the fastest electron plasma waves down to the slow fluctuations due to ion and atom inertial effects. The common theme of this study is flows, nonlinear waves and instabilities in low-temperature plasmas with atomic processes such as ionization and instabilities. Several nonlinear plasma problems related to applications in electric propulsion and open-mirror linear fusion devices are studied in this thesis. Hall thrusters, the devices for electric propulsion, are prone to many waves and instability phenomena, and the low-frequency ionization oscillations (propagating along its channel) stand as most commonly observed (so-called breathing mode). Though the ionization nature of the breathing mode is generally accepted, with the mode frequency scaling as the fly-by time of the slow neutral atoms, exact mechanisms remain poorly understood. In this study, we formulate a full fluid model for three species: atoms, ions, and electrons, and perform a comprehensive benchmark study between the fluid model and hybrid model (heavy kinetic species and fluid electrons). A novel result of this study is the identification of two different regimes of breathing modes. In one regime, the breathing mode co-exists with the higher frequency resistive mode, and the second - is clear breathing mode. The main features and characteristics of these regimes are identified and confirmed in both models. Generally, the benchmark study shows a good agreement between the fluid and kinetic models. A simple reduced fluid model is proposed for the solo regime. In this regime, the ion backflow region (the near-anode region with negative ion velocity) is identified as a driving mechanism for the breathing mode.
The related theme of this work is the role of atomic physics effects (ionization and charge exchange) on plasma flow in the divertors of linear fusion devices. In open magnetic field configurations, the magnetic mirrors are placed at the ends both to confine the plasma in the core and to distribute output energy over a larger area, thus reducing the wall load. Direct interaction of plasma flow with the material wall results in the re-emission of neutrals into the plasma (recycling) due to particles' reflection, desorption, and other processes.
This re-emitted neutral component can dramatically impact the whole system. It is found that the low-energy neutral component has the largest influence, generating ion sources (via ionization and charge exchange) in the region near the wall and resulting in strong modification of plasma potential
and flow. To study these effects, we have developed a time-dependent hybrid drift-kinetic code with a detailed model of atom transport near the wall, including collision processes. This tool can be used for studying global quasineutral plasma flow dynamics and its interaction with atom components, such as in divertors of linear fusion devices. To illustrate its capabilities, we confirm previous findings (based on qualitative and steady-state analysis) that the ion temperature in the source generally reduces the transport of neutral atoms. Additionally, we show that an increase in the density of slow atoms above some critical value results in dramatic destabilization of plasma flow via ion streaming instabilities.
Description
Keywords
plasma physics, electric propulsion, Hall thruster, low-frequency modes, plasma modeling, magnetic nozzle
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Physics and Engineering Physics
Program
Physics