Ph.D. Thesis Project: Thermal Waves and Turbulent Structures in Magnetized Plasmas

I completed my Ph.D. thesis in 2009. Copies of it are available in both PDF and HTML format. The PDF is the source for the printed copies that were turned in to the UCLA Library as part of the requirements for completion. The web version omits the front matter but does present every chapter and the full appendices. A link to the downloadable PDF is provided below, along with the Table of Contents for the web version.

General Information

Title: Spontaneous Thermal Waves and Exponential Spectra Associated with a Filamentary Pressure Structure in a Magnetized Plasma
Department: Physics and Astronomy
Institution: University of California, Los Angeles


Table of Contents

Abstract

1 Introduction

2 Experimental Setup and Overview of the Temperature Filament

3 Identification of a Spontaneous Thermal Wave

4a Exponential Frequency Spectrum and Lorentzian Pulses (Part 1)

4b Exponential Frequency Spectrum and Lorentzian Pulses (Part 2)

5 Comparison Between Temperature Filament and Limiter-edge Experiments

6 Plasma Flow Parallel to Background Magnetic Field

7 Conclusions

Appendix A Wavelet Analysis to Calculate Power Spectra

Appendix B Pulse Detection Techniques

Bibliography

Abstract

An experimental study of plasma turbulence and transport is performed in the fundamental geometry of a narrow pressure filament in a magnetized plasma. An electron beam is used to heat a cold background plasma in a linear device, the Large Plasma Device (LAPD-U) [W. Gekelman et al. Rev. Sci. Instrum. 62, 2875 (1991)] operated by the Basic Plasma Science Facility at the University of California, Los Angeles. This results in the generation of a filamentary structure (~ 1000 cm in length and 1 cm in diameter) exhibiting a controllable radial temperature gradient embedded in a large plasma. The filament serves as a resonance cavity for a thermal (diffusive) wave manifested by large amplitude, coherent oscillations in electron temperature. Properties of this wave are used to determine the electron collision time of the plasma and suggest that a diagnostic method for studying plasma transport can be designed in a similar manner. For short times and low heating powers the filament conducts away thermal energy through particle collisions, consistent with classical theory. Experiments performed with longer heating times or greater injected power feature a transition from the classical transport regime to a regime of enhanced transport levels. During the anomalous transport regime, fluctuations \r\nexhibit an exponential power spectrum for frequencies below the ion cyclotron frequency. The exponential feature has been traced to the presence of solitary pulses having a Lorentzian temporal signature. These pulses arise from nonlinear interactions of drift-Alfvén waves driven by the pressure gradients. The temporal width of the pulses is measured to be a fraction of a period of the drift-Alfvén waves. A second experiment involves a macroscopic (3.5 cm gradient length) limiter-edge geometry in which a density gradient is established by inserting a metallic plate at the edge of the nominal plasma column of the LAPD-U. In both experiments the width of the pulses is narrowly distributed, resulting in exponential spectra with a single characteristic time scale. The temperature filament experiment permits a detailed study of the transition from coherent to turbulent behavior and the concomitant change from classical to anomalous transport. In the limiter experiment the turbulence sampled is always fully developed. The similarity of the pulse shapes and fluctuation spectra in the two experiments strongly suggests a universal feature of pressure-gradient driven turbulence in magnetized plasmas that results in non-diffusive cross-field transport. This may explain previous observations in helical confinement devices, research tokamaks and arc-plasmas.

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