Microwave Loss in a Reflex Plasma Discharge

Abstract

The Q of a partially ionized plasma with fluctuating density, created by a reflex discharge, is determined by measuring the decay of microwave energy in a cavity coaxial to the plasma column. For the temperature and neutral gas pressure range of the experiment (Te≃5 eV, p≃5×10−4 Torr for a hot-cathode discharge) the experimental results agree with the theory that electron-neutral collisions dominate below a few percent ionization, and electron-ion collisions become important at higher ionizations. For cold-cathode discharges, with higher temperature, the electron-ion collisions are not significant. Virtual collisions with the ends of the interaction region also contribute to the loss for both hot and cold cathode discharges. Anomalous diffusion does not play a significant role in the microwave losses. The absolute magnitude of the experimentally determined collision time is lower than that calculated from the theory by a factor of three to four, which is in agreement with other measurements reported in the literature. For cold-cathode operation there remain some unexplained inconsistencies within the experimental results.