マイクロプラズマスラスターの研究開発

抄録

This paper proposes an electrothermal microplasma thruster using azimuthally symmetric microwave-excited plasmas, which consists of a microplasma source and a micronozzle. The microplasma source is made of a 10 mm long dielectric chamber of 2 mm in inner diameter covered with an electrically grounded metal, which produces high temperature plasmas at around atmospheric pressure. The micronozzle has a throat of 0.2 mm in diameter, which converts high thermal energy of plasmas into directional kinetic energy to produce the axial thrust. First, we have developed a numerical model for Ar microplasmas and micronozzle flows to estimate the thruster performance. The model consists of three modules: a volume-averaged global model, an electromagnetic model for microplasma sources, and a fluid model for micronozzle flows. Numerical results indicate that the microwave power absorbed in plasmas increases with microwave frequency f and relative permittivityε_d of the dielectric chamber, to achieve the plasma density in the range 10¹⁴-10¹⁶ cm^-3. A certain combination of the frequency and permittivity significantly increases the power absorption. 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 effect 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 (P_t <10 W), which is applicable to a station-keeping maneuver for microspacecraft less than 10 kg. Moreover, we have developed a microwave-excited microplasma source, which has a dielectric chamber of 10 mm length and 1.5 mm in inner diameter, where off-the-shelf mullite (ε_d ≈ 6) and zirconia (ε_d ≈ 12-25) tubes are employed. Experiments were performed at f = 2 and 4 GHz, P_t < 10 W, an Ar flow rate of 50 sccm, and a microplasma pressure of 10 kPa, where optical emission spectroscopy and Langmuir probe measurement were employed for the diagnostics of microplasmas. The measurements indicate that the Ar I emission intensity and plasma density n_e increase with f and ε_d, and that the n_e is in the range 10¹¹-10¹³ cm^-3. The rotational temperature T_rot of N₂ molecule in the added gas was in the range 1100-1500 K, and the specific impulse estimated from the temperature T = T_rot was determined to be approximately 70 s from the model analysis.