Imaging and Probe Techniques for Wave Dispersion Estimates in Magnetized Plasmas

Light, Adam David

Graduate Thesis Or Dissertation

Imaging and Probe Techniques for Wave Dispersion Estimates in Magnetized Plasmas Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/q237hr95t

Abstract

Fluctuations in magnetized laboratory plasmas are ubiquitous and complex. In addition to deleterious effects, like increasing heat and particle transport in magnetic fusion energy devices, fluctuations also provide a diagnostic opportunity. Identification of a fluctuation with a particular wave or instability gives detailed information about the properties of the underlying plasma. In this work, diagnostics and spectral analysis techniques for fluctuations are developed and applied to two different laboratory plasma experiments. The first part of this dissertation discusses imaging measurements of coherent waves in the Controlled Shear Decorrelation Experiment (CSDX) at the University of California, San Diego. Visible light from ArII line emission is collected at high frame rates using an intensified digital camera. A cross-spectral phase technique allows direct visualization of dominant phase structures as a function of frequency, as well as identification of azimuthal asymmetries present in the system. Experimental dispersion estimates are constructed from imaging data alone. Drift-like waves are identified by comparison with theoretical dispersion curves, and a tentative match of a low- frequency spectral feature to Kelvin-Helmholtz-driven waves is presented. Imaging measurements are consistent with previous results, and provide non-invasive, single-shot measurements across the entire plasma cross-section. Relationships between imaging and electrostatic probe measurements are explored. The second part of this dissertation discusses the design and construction of diagnostics for the Colorado Field-Reversed Configuration (CFRC), as well as preliminary results. A triple probe, a Mach probe, a multi-chord heterodyne interferometer, Rogowski coils, a single-point, three-axis magnetic probe, and a 16-point, three-axis magnetic probe have been constructed. Each diagnostic is designed for fluctuation measurements up to the data acquisition Nyquist frequency of 20 MHz. A histogram cross-spectral analysis technique allows experimental dispersion estimates to be made from multi-channel magnetic measurements. Hints of waves in the range of ion-cyclotron frequency harmonics are observed, but lack of global information about the plasma objects formed in CFRC prevents definitive interpretation.

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