A miniature electrothermal thruster using microwave-excited plasmas: a numerical design consideration

A miniature electrothermal thruster has been proposed using azimuthally symmetric microwave-excited plasmas, and numerical investigations have been conducted for design consideration. The microthruster consists of a microplasma source and a micronozzle. The former, made of a dielectric chamber 1 mm in radius and 1 0mm long covered with a grounded metal, produces high temperature plasmas in Ar at around atmospheric pressures. The latter converts such high thermal energy into directional kinetic energy through supersonic nozzle expansion to obtain the thrust required. The numerical model consists of three modules: a global model and an electromagnetic model for microplasma sources and a fluid model for micronozzle flows. Simulation was conducted separately for the plasma source and nozzle flow. The numerical results indicated that the microwave power absorbed in plasmas increases with increasing microwave frequency and relative permittivity of dielectrics, to achieve plasma density in the range 10(19) - 10(22) m(-3), electron temperature in the order of 10(4) K and heavy particle temperature in the range 103 - 104 K at a microwave input power of <= 10W; in practice, surface waves tend to be established in the microplasma source at high frequencies and permittivities. A certain combination of frequency and permittivity was found to significantly enhance the power absorption, enabling the microplasma source to absorb almost all microwave input powers. Moreover, the micronozzle flow was found to be very lossy because of high viscosity in thick boundary layers, implying that shortening the nozzle length with increasing half-cone angles suppresses the effects of viscous loss and thus enhances the thrust performance. A thrust of 2.5 - 3.5 mN and a specific impulse of 130 - 180 s were obtained for a given microwave power range of interest, which is applicable to a station-keeping manoeuvre for microspacecraft less than 10 kg.