Low-power microwave arcjet testing - Plasma and plume diagnostics and performance evaluation

The results of an experimental study of a microwave thruster are presented. Performance evaluation of a 7.5 GHz engine was done using different propellants under both atmospheric and vacuum conditions. Helium, nitrogen and ammonia were tested providing mean chamber stagnation temperature values. Other experiments include plasma and plume diagnostics. Emission spectroscopy of the plasma was made in order to measure the plasma electron temperature at different specific power levels, and the commonly-made assumption of Local Thermodynamic Equilibrium (LTE) was examined. In order to obtain thrust and specific impulse data under vacuum conditions, the Doppler shift between light emitted by the exhaust plume parallel to the gas velocity and perpendicular to the flow was measured using a high spectral resolution Fabry-Perot interferometer, yielding centerline specific impulse values for helium propellant at various specific powers. Thrust and mean specific impulses for all three propellants are being measured using a vertical mechanical thrust stand mounted inside a vacuum tank. Nomenclature