The experimental development of a microwave resonant cavity electrothermal thruster

An experimental investigation of the properties and behavior of microwave resonant cavity plasmas has been conducted with the objective of developing a microwave electrothermal thruster. Both the TM₀₁₁ and the TM₀₁₂ resonant cavity modes have been used with helium and nitrogen gases to produce axially symmetric plasmas which are located away from the solid surfaces of the containment vessel. Comparisons with a companion numerical study have also been made in both of the cavity modes. Free floating plasmas in the TM₀₁₂ mode exhibited electromagnetic coupling efficiencies of up to 80% and could be sustained up to 330 kPa. The incident power to the cavity was, however, limited to about 500 W as a result of perturbations in the cavity electromagnetic fields arising from an asymmetric introduction of the microwave power. Incident powers greater than 500 W resulted in an excursion of the plasma to the surface of the quartz containment vessel. Bluff-body and swirling flow stabilization schemes were used in an attempt to overcome this stability problem. Swirling flows were ineffective, but bluff-body stabilization proved to be very successful. Bluff-body stabilized plasmas in the TM₀₁₁ mode were investigated with powers and pressures of up to 2.25 kW and 500 kPa and exhibited electromagnetic coupling efficiencies approaching 100%. The maximum calculated specific impulse and overall efficiency of this system were 543 s and 68.9%, respectively. A spectroscopic analysis yielded very flat radial electron temperature profiles, with peak values ranging from 10,200 K to 10,900 K for helium plasmas in the TM₀₁₂ mode and from 11,840 K to 12,170 K in the TM₀₁₁ mode. Heavy particle temperature measurements were similar to the electron temperatures, but were not sufficiently precise to determine whether or not the plasma is in local thermodynamic equilibrium.