Studies of dielectric multipactor discharges and gas breakdown initiated by high power millimeter waves

Abstract

This thesis reports the theoretical and experimental investigation of discharge phenomena induced at dielectric surfaces and in gases by a 1.5 MW, 110 GHz gyrotron operating in 3 [mu]s pulses. The experimental study of multipactor discharges on dielectric surfaces in vacuum was performed at 110 GHz, a frequency that is an order of magnitude higher than in previous experimental studies. Two separate test assemblies were constructed: one with the 110 GHz electric field tangential to the dielectric surface and the second with a perpendicular field. Threshold electric fields for multipactor onset were measured in both field orientations for samples of sapphire, alumina, fused quartz, crystal quartz and silicon. The present results at 110 GHz, when combined with previously published data at lower frequencies, show that the threshold electric field for initiation of a dielectric multipactor discharge scales linearly with frequency.

This linear scaling, which is favorable for operation at high frequency, is in good agreement with theoretical predictions. The absorbed RF power due to multipactor was also measured as a function of RF intensity. At low intensities, absorbed RF power was found to agree quantitatively with theoretical predictions, though experimental results diverged from theory at higher RF intensities. Calculations of dielectric multipactor were carried out to help analyze the experimental results. A new equation for predicting threshold RF electric fields was derived that agrees with a wide variety of existing experimental data, spanning orders of magnitude in frequency and a variety of geometries and materials. The theory work also suggests strategies for mitigation of multipactor, dependent upon experimental geometry. Gas breakdowns were experimentally characterized at both 110 and 124.5 GHz by focusing the gyrotron beam in air at pressures from 25 to 760 Torr.

Prior studies of this system had revealed that the plasma spontaneously forms a reproducible twodimensional array of filaments. Optical emission spectroscopy was used to measure peak electron density in this plasma, through Stark broadening of the H[subscript alpha] line. Heating dynamics of the background neutral gas were spatially and temporally resolved using two-dimensional laser interferometry. These results hell) test the theory of gas breakdown at 110 GHz.