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Microwave plasma diagnostics Shankowski, Allison Eugene

Abstract

This thesis is concerned mainly with developing accurate microwave methods for determining electron-density distributions in transient plasma columns. Two new methods have been developed and these have been evaluated by comparing with other methods. The first is a microwave refraction technique which depends on multiple reflections of an obliquely-incident microwave beam between the plasma critical layer and the walls of the discharge tube. It offers several advantages over previous techniques using microwave refraction; it is simpler and faster to implement, and yields accurate and more reliable results. Several methods for reconstructing profiles from refraction data are described. The second method depends on reflection measurements at normal incidence, and is suitable for plasmas in which the maximum plasma frequency is greater than the applied frequency. This method is based on measuring the rate of change in phase of the reflected signal due to Doppler shift in frequency produced by motion of the plasma critical layer. It has the advantage that the required data can be obtained from relative phase measurements which are less subject to experimental error than absolute phase measurements. Several methods of reconstructing profiles are developed. Approximate profiles can be determined from data at two frequencies only, while more detailed profiles involve applying a step-by-step procedure to data obtained at several frequencies. The sensitivity of the method to certain experimental errors is investigated and the validity of ray theory in the normal-incidence application is examined. A few related measurements were carried out in order to examine the validity of some assumptions made in connection with the microwave methods for determining profiles. The effect of polarization, the effect of altering the discharge-current waveform, and the possibility of a density gradient in the axial direction were investigated. Iangmuir double probes were used in a few cases to obtain an independent measurement of the electron-density distribution. Using the microwave refraction technique, extensive measurements were carried out in an afterglow plasma for various discharge conditions. A few interesting characteristics were observed, and a possible reason for the observed shape of electron-density profiles is suggested.

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