Preliminary Investigation of High Power Microwave Plasmas for Electrothermal Thruster Use

EZ, E,^, EP Results are reported from preliminary tests to evaluate the high power microwave electrothermal thruster (MET) fr concept, which employs a free-floating plasma discharge maintained by applied continuous wave (CW) microwave Hz, H,^, HP power to heat a propellant gas flow. Stable plasmas were created and maintained in helium (He), nitrogen (N 2), and hydrogen (H2) as propellants in both the TMOI I and TM012 h modes at discharge pressures from 10 Pa to 69 kPa. Reproducible starting conditions of pressure and power were I documented for all the plasmas. Vortical inflow of the propellant gas was observed to cause the formation of onJo, Jl axis "spike" plasmas. The conditions under which these plasmas form and unform were studied. Operation in the me spike plasma condition enables maximum power absorption with minimum wall heating and offers maximum efficiency ne in heating the propellant gas. In the spike condition, plasmas of the three propellant gases were investigated in an open P channel configuration to a maximum applied power level of 11.2 kW (in N2). Microwave power coupling efficiencies p over 90 percent were routinely obtained at absorbed power levels up to 2 kW. Magnetic nozzle effects were investigated using a superconducting solenoidal magnet to apply p a high magnetic field to the plasmas in, and exiting from, the discharge tube. q