| The microwave electrothermal thruster (MET) uses microwave frequency energy to create and sustain a resonant cavity plasma to heat a propellant. It has been operated at a variety of power levels with several propellants. The performance potential of the device has not previously been ascertained because of complex physics involved in the microwave heating, the relatively low thrust of the device, and difficulty in using conventional diagnostics to study molecular plasmas. The objectives of this investigation were to measure heavy particle temperatures and to understand gas heating processes in the MET plasma chamber for representative molecular propellants, oxygen and nitrogen. These molecules have well known thermochemical and structural properties, and they are components of liquid-storable propellants. A 2.45 GHz aluminum cylindrical thruster with converging copper alloy nozzles was used. A spectroscopic system was used to collect light emitted through a window in the plasma chamber. A Schumann-Runge emission model was developed assuming anharmonically vibrating, non-rigid rotating oxygen molecules. The commercially available LIFBASE software was used to model ionized molecular nitrogen first negative system emission from nitrogen plasmas. Experimental data were compared to models using least squared difference summation schemes.; Steady and repeatable plasmas were formed with oxygen, nitrogen, and ammonia for most operating conditions. Strong coupling between fluid dynamics and plasma geometry was observed for high flow rate nitrogen tests. Oxygen temperatures of 2,000 K were measured with no variation due to spatial location or pressure and a slight increase in temperature with specific absorbed power. Nitrogen temperatures of 5,500 K were measured with no variation due to location, pressure, or specific absorbed power. Thermochemical calculations show the relationship between equilibrium enthalpy addition, temperature, dissociation fraction, and specific impulse. Nitrogen was found to be an excellent choice as a propellant component while oxygen was found to be a poor choice. |
