Discharge Modes Observed in Plasmas Sustained in Focused Microwave Beams

Plasma discharges are sustained in the focus of a continuous, high-power microwave beam. The experimental design minimizes interactions between the chamber walls and both the microwave beam and the resulting discharges so as to approximate free-space conditions. Discharges were generated by a multikilowatt, 4.7-GHz CW microwave beam at pressures ranging from 100 to 200 mtorr. The CW source enabled the evolution of the plasma-beam system to be observed over time scales ranging from seconds to hours. A variety of discharge behaviors are observed depending on the conditions (power, pressure, and gas composition) within the chamber. The observed discharges can be classified into three discharge modes: unstable, stable, and quasi-stable. Here, we present observations and a preliminary analysis of the various discharge modes observed in the experiment.