An Investigation of the Stability and Characteristics of Plasma Generated at the Focus of a Continuous-Wave Microwave Beam

Abstract

Developing accurate simulations requires benchmarking numerical codes against high-quality experimental data. To validate and improve theoretical models of microwave-plasma interactions in free space, the Air Force Research Laboratory (AFRL) designed and constructed an experimental setup to study plasma generated at the focus of a high power continuous-wave (CW) microwave beam. Discharges were sustained in the center of a vacuum chamber far removed from the chamber walls, which helped prevent non-ideal effects, such as contaminants, secondary electron emission, and plasma-induced desorption, from influencing the properties and behavior of the discharge. A thorough description of the behavior and characteristics of the discharges produced in the AFRL’s experimental setup is the first step towards producing the high-quality data needed. Experiments were conducted with gas mixtures of argon, nitrogen, and oxygen at gas pressures ranging from 100 to 200 mTorr. Three types of discharges were observed during these experiments: unstable, quasi-stable, and stable discharges. It was determined that the stability of the discharges could be controlled through adjustments of the gas composition, gas pressure, and power of the microwave beam; the operational conditions in which each of the discharges was observed was documented. Invasive (electrostatic probe) and non-invasive (optical emission spectroscopy) plasma diagnostic methods were implemented to characterize the discharges generated in the experimental setup. From these studies, it was determined that quasi-stable discharges were driven by instabilities resulting in the periodic propagation of ionization fronts. In stable discharges, xix striations were observed in all tested conditions; up to three different striation patterns were observed simultaneously. An anticorrelation of the electron temperature and density along the path of the striations was observed in the measurements of the plasma parameters; this is consistent with behavior reported in the literature of striations in DC, and RF discharges. While the source of the striations is still not fully understood, experimental and simulation results suggest that standing waves within the discharge might be the underlying cause of the striated patterns. Experimental measurements of the electron temperature and density showed similar trends in all tested gas mixtures (Ar-N2, Ar-O2, and Ar-N2-O2) as the gas pressure, the concentration of the molecular gas and power of the microwave beam were varied. However, there was disagreement in the electron temperatures measured by the invasive and non-invasive diagnostic systems in discharges containing oxygen. GlobalKin, a zero-dimensional plasma kinetics model, was also used to study the plasma parameters of the microwave-driven discharges. Results from the simulations closely agreed with the electron temperatures measured by the non-invasive method, suggesting the presence of oxygen in the discharge affected the measurements of the invasive method. In general, GlobalKin simulations showed good agreement with experimental results.