Conductance Peaks at the Cyclotron Harmonics in a Cylindric Plasma Capacitor

Citation

Downward, James Germain, IV (1971) Conductance Peaks at the Cyclotron Harmonics in a Cylindric Plasma Capacitor. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4GQW-D336. https://resolver.caltech.edu/CaltechTHESIS:04182018-145000604

Abstract

The conductance peaks at the cyclotron harmonics in a plasma­ filled cylindrical capacitor are investigated theoretically and experimentally. A theoretical model is developed and is shown to predict peak height variation with density, harmonic number, and sheath thickness which is in agreement with experimental measurements of the capacitor's complex admittance (susceptance and conductance) and absorption coefficient. A harmonic conduction peak is found to rise initially as density increases, only to fall to a minimum height as the upper hybrid frequency passes through that harmonic. The noise output of the capacitor is studied and is compared to the theoretical noise output at the harmonics which would be expected for a thermal plasma. The theoretical output is shown to be related to the capacitor's conductance. The capacitor's internal noise oscillations are found to be too intense for a thermal plasma, but the noise peaks at the harmonics show structure and amplitude variation with density in substantial agreement with the theoretical model. To test the validity of the theoretical model which treats the capacitor's center wire sheath as a vacuum region several Debye lengths thick, the conductance change for small decreases in sheath size is investigated both experimentally and theoretically. It is found that the amplitude of the conductance peak within a harmonic passband is very sensitive to small changes in sheath size. This fact leads to a new plasma diagnostic, sheath modulated transmission. Sheath modulated transmission experiments are found to enhance cyclotron harmonic wave effects relative to the amplitude of the transmission peaks at harmonics and to provide information as to the location of the second harmonic band pass edge. Applications of this work to plasma diagnostics and suggestions for further work are made.

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