Plasma and aerodynamic features have been investigated for a microplasma thruster of electrothermal type using azimuthally symmetric microwave-excited microplasmas. The thruster developed consisted of a microplasma source 1.5 mm in diameter, 10 mm long with a rod antenna on axis, and a converging-diverging micronozzle 1 mm long with a throat 0.2 mm in diameter. The feed or propellant gas employed was Ar at pressures of 10-50 kPa with flow rates of 10-70 SCCM ͑SCCM denotes standard cubic

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... er per minute at STP͒ and the surface wave-excited plasmas were established by 4.0 GHz microwaves at powers of Յ6 W. Numerical analysis was made for the plasma and flow properties by developing a self-consistent, two-dimensional model, where a two-temperature fluid model was applied to the entire region through the microplasma source to the micronozzle ͑or through subsonic to supersonic͒; in the former, an electromagnetic model based on the finite difference time-domain approximation was also employed for analysis of microwaves interacting with plasmas. In experiments, optical emission spectroscopy was employed with a small amount of additive gases of H 2 and N 2 , to measure the plasma electron density and gas temperature in the microplasma source around the top of the microwave antenna, just upstream of the micronozzle inlet; in practice, the numerical analysis exhibited a maximum thereabout for the microwave power density absorbed, plasma density, and gas temperature. The Stark broadening of H Balmer line and the vibronic spectrum of N 2 second positive band indicated that the electron density was in the range of ͑3-12͒ ϫ 10 19 m −3 and the gas or rotational temperature was in the range of 700-1000 K. The thrust performance was also measured by using a microthrust stand with a combination of target and pendulum methods, giving a thrust in the range of 0.2-1.4 mN, a specific impulse in the range of 50-80 s, and a thrust efficiency in the range of 2%-12%. These experimental results were consistent with those of numerical analysis, depending on microwave power and gas flow rate.