Resonance Cones in the Field Pattern of a Short Radio Frequency Probe in a Warm Anisotropic Plasma
An experimental investigation of the angular field pattern of a small radio frequency probe in a plasma in a magnetic field is described. The field is observed to become very large along a resonance cone whose axis is parallel to the static magnetic field and whose opening angle is observed to vary with incident probe frequency, electron cyclotron frequency, and electron plasma frequency in agreement with simple cold plasma dielectric theory. The use of the resonance cone angle as a diagnostic tool to measure the plasma density in a plasma in a magnetic field is discussed. It is noted that similar cones might be expected near the ion cyclotron frequency. The relationship of these cones to the limiting phase- and group-velocity cones which appear in the theory of plane wave propagation is discussed. The necessity for examining the allowed directions of the group velocity rather than the allowed directions of the phase velocity and customary phase velocity plots when determining whether propagation between two remote points in a plasma is possible, is emphasized. The addition of electron thermal velocities to the theory is examined in the limit of a large static magnetic field. The resonance cone angle is shifted to a slightly smaller angle than that predicted by cold plasma theory, and a fine structure appears inside the cones and is shown to result from an interference between a fast electromagnetic wave and a slow plasma wave. The interference structure is observed experimentally, and measurements of the angular interference spacing are shown to agree with the warm plasma theory.
The author wishes to express his deep appreciation to his advisor, Professor Roy W. Gould, for his suggestion of the problem and his continued guidance, encouragement and constant interest throughout the course of this research. His many direct contributions to this work are gratefully acknowledged. The author is also indebted to Mr. Nagendra Singh for his work on some of the theoretical aspects of this research. Many members of the Plasma Laboratory contributed directly or indirectly to this work, and particular thanks go to Mr. James Downward for many helpful discussions and for the use of his apparatus during the initial stages of this research, Dr. Reiner Stenzel and Professor Robert S. Harp for their very helpful comments and advice, and Mr. Charles Moeller for his help in the microwave interferometer measurements. Special thanks are extended to Mrs. Ruth Stratton for her excellent typing of the manuscript. The author gratefully acknowledges the generous financial support he received from the National Science Foundation during the course of his graduate work. This investigation was supported in part by the United States Office of Naval Research and in part by the United States Atomic Energy Commission. This work is dedicated to the author's wife, Sally, whose love, patience, encouragement and understanding have been helpful in many ways.
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