Plasma Diagnostics and Thrust Performance Analysis of a Microwave-Excited Microplasma Thruster

A microwave-excited microplasma source for a miniature electrothermal thruster has been investigated by optical emission spectroscopy and an electrostatic probe. The microplasma source is made of a dielectric tube 10 mm long and 1.5 mm in inner diameter, producing high temperature Ar plasmas in the pressure range from 5 to 40 kPa. Plasma diagnostics showed that higher microwave frequencies and dielectric constants resulted in desirable plasma characteristics: electron densities of 1017–1019 m-3 and rotational temperatures of 700–1800 K at microwave powers below 10 W. Moreover, the temperature increased toward the exit of the plasma chamber where a micronozzle is equipped. Numerical analysis of thrust performance based on the plasma diagnostics indicated that smaller throat diameters of the micronozzle produced better thrust performances: thrusts of 0.98–1.2 mN and specific impulses of 67–81 s.