Finite difference simulations of neutral gas-MHD interactions in partially ionized plasmas

Wilson, Alasdair David (2016) Finite difference simulations of neutral gas-MHD interactions in partially ionized plasmas. PhD thesis, University of Glasgow.

Abstract

This thesis deals with the theoretical and numerical modelling of partially ionized plasmas. The study of partially ionized plasmas is important in both astrophysical and laboratory contexts and we present a novel finite difference approach to modelling a magnetohydrodynamic plasma and hydrodynamic gas as well as some interaction terms between them. In particular we model fluid limits of a collisional drag, momentum coupling term and a critical velocity (Alfv ́en ) ionization term. Chapter 1 reviews the necessary background material relevant to this thesis. We introduce relevant plasma parameters that are useful to help understand the regimes at which MHD operates and which are later used when placing criteria on the conditions required for, and the rate equations of, ionization. We introduce the fluid limit model of non-resistive plasmas (ideal MHD), as well as theoretical models for gas-plasma collisions and Alfv ́en ionization.
Chapter 2 lays out the model of a linear finite difference gas-MHD momentum
coupling code (GMMC) that we modify by the addition of a fluid Alfv ́en ionization term. We explore the codes stability and fidelity and we explore Fresnel interference patterns as a test scenario.
Chapter 3 lays out the model equations and derivation of a non-linear finite difference gas-MHD interactions code (GMIC). Chapter 4 uses the code GMIC to explore the momentum coupling between the gas and plasma fluids in a non-linear regime. We show that the presence of a frictional drag term effects both fluids in a variety of simulated scenarios. Propagation of waves and diffusion are effected significantly across a range of parameter space. We show theformation of ‘plasmoids’ by interacting, momentum coupled waves. We see that the momentum coupling has a somewhat similar effect on plasmas as a resistive term does.
Chapter 5 uses both the linear and non-linear codes to simulate Alfv ́en ionization in a variety of scenarios. With both codes we see that waves and flows can be sources of ionization of the relative velocity between the two simulated fluids exceeds a pre-set threshold. This ionization effects the dynamics of the system significantly, firstly the extraction of kinetic energy in order to ionize introduces a directional and amplitude dependant damping of waves; secondly by providing a source of new plasma that the fluid is forced to react to. Also in this chapter we discuss how Alfv ́en ionization might play a role in astrophysical contexts, in particular, the solar photosphere and brown dwarf atmospheres.
Chapter 6 deviates from the previous work to explore the possibility that dielec-trophoretic forces may be able to stratify the dynamic atmosphere of the Sun. We derive a simple expression for the DEP-force due to a neutral particle moving through a magnetic field and becoming polarized and we examine the displacement caused by this field and the ambipolar field. We also simulate the ability of a hypothetical DEP-force to separate elements based on their polarizability to mass ratio.
Chapter 7 summarises the conclusion of the previous chapters and includes a brief discussion about possible extensions to this work.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: plasmas, partially-ionized plasmas, MHD, two-fluid plasmas, critical velocity ionization, Alfven ionization, dielectrophoresis, fluid dynamics, finite-difference, solar, brown dwarfs.
• Subjects: Q Science > QB Astronomy; Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Funder's Name: Science & Technologies Facilities Council (STFC)
• Supervisor's Name: Diver, Dr. Declan A.
• Date of Award: 2016
• Depositing User: Mr Alasdair D. Wilson
• Unique ID: glathesis:2016-7209
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 03 May 2016 09:15
• Last Modified: 18 May 2016 08:20
• URI: https://theses.gla.ac.uk/id/eprint/7209