Investigation of free-floating resonant cavity microwave plasmas forpropulsion

Results of experiments with high-pressure helium and nitrogen discharges generated in a microwave resonant cavity for use in an electrothermal thruster are presented. The cavity, operating in the TMoii 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 2.79 x 10 4 kg/s. Coupling efficiencies up to 19% have been demonstrated, and temperature measurements 200 mm downstream of the plasma indicate thermal efficiencies of up to 36.6% and total efficiencies of up to 25.2%, both increasing linearly with mass flow rate. The downstream temperature measurements also closely match the coupling efficiency behavior when plotted against pressure. Absolute measurements of the continuum radiation yield electron temperatures of between 10,200 and 10,900 K, which are insensitive to changes in the operating conditions.