S M. Finnegan

Date of Graduation

2008

Document Type

Dissertation/Thesis

Abstract

In this thesis, a non-linear, collisional, two-fluid model of uniform plasma convection across field-aligned current (FAC) sheets, describing stationary Alfven (StA) waves is developed in support of laboratory experiments performed to test the hypothesis that a stationary inertial Alfven wave pattern forms within a channel of parallel electron current across which plasma is convected. In a previous work, Knudsen (D. J. Knudsen, J. Geophys. Res. 101, 10,761 (1996)) showed that, for cold, collisionless plasma, stationary inertial Alfven (StIA) waves can accelerate electrons parallel to a background magnetic field and cause large, time-independent plasma-density variations having spatial periodicity in the direction of the convective flow over a broad range of spatial scales and energies. Here, Knudsen’s model has been generalized for warm, collisional, anisotropic plasma. The inclusion of parallel electron thermal pressure introduces dispersive effects which extend the model to the kinetic (β > me/mi) regime. The effects of both ion-neutral and electron-ion collisional resistivity on StIA and stationary kinetic Alfven (StKA) wave solutions is studied. Conditions for both periodic and solitary wave solutions are identified. In the small amplitude limit, it is shown that the StA wave equation reduces to the differential equation describing the behavior of a forced harmonic oscillator. Analytical solutions are obtained for both a step and impulse, of finite width, forcing functions. Plasma rotation experiments in the West Virginia University Q-machine (WVUQ) demonstrate that an electron-emitting spiral electrode produces controllable, parabolic radial profile of floating potential, while the space potential showed no such structure. Laser-induced fluorescence measurements confirmed that the azimuthal ion drift velocity is inconsistent with a drift due to a gradient in the space potential. Experiments designed to produce StIA wave signatures were performed in the afterglow plasma of the LArge Plasma Device Upgrade (LAPD-U) at the University of California Los Angeles’s (UCLA’s) Basic Plasma Science Facility (BAPSF). Alfven waves launched within the current channel (depleted density) were confined with the channel and more generally demonstrated the ability to produce localized IAW in the plasma afterglow. When electron current (density depletion) and cross-field plasma flow are combined, time-stationary, self-excited, density waves which are collocated with the current channel are observed. Excellent qualitative agreement between the characteristics of the observed density structures and the predictions from the collisional StIA wave model, lead to the conclusion that the measured density structures are the first experimental observations verifying the existence of StIA waves.

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