Investigation of free-floating nitrogen and helium plasmas generated in a microwave resonant cavity

Results of experiments with high pressure helium and nitrogen discharges generated in a microwave resonant cavity are presented. The cavity, operating in the TM(012) mode, generated the discharges within a quartz sphere which allowed the discharge to be both free-floating and away from solid surfaces. Input powers of up to 400 W were used with gas pressures up to 300 kPa (absolute) and mass flow rates up to 0.004.63 kg/s. Coupling efficiencies up to 79 percent have been demonstrated, and temperature measurements 200 mm downstream of the plasma indicate energy absorption of up to 47 percent by the flowing gas. The downstream temperature profiles also closely match the coupling efficiency profiles, showing a peak when plotted against pressure. Absolute measurements of the continuum yield electron temperatures of between 10,200 K and 10,900 K, which are insensitive to changes in the experimental parameters.