Microplasma thruster powered by X-band microwaves

Abstract

A microplasma thruster of the electrothermal type has been developed with X-band (f = 11 GHz) microwaves and Ar as a propellant. The emphasis was placed on an understanding of distinguished features of the microplasma generation and thrust performance by X-band microwaves, compared with those by S-band (f = 4 GHz) ones. The thruster consisted of a microplasma source 2 mm in the inner diameter and 3–12 mm long with a rod antenna on the axis, followed by a converging-diverging micronozzle. Azimuthally symmetric surface wave-excited plasmas were established by microwaves at powers of ≤6 W, with the source pressure in the range 2–50 kPa at flow rates of 10–70 sccm. The plasma generation, nozzle flow, and thrust performance were numerically analyzed using a two-dimensional fluid model, coupled with an electromagnetic model for microwaves interacting with plasmas in the source region. Simulations indicated that higher frequency f = 11 GHz with the source chamber length Lₛ≈ 1/4 of the driving wavelength λ is preferred for the microplasma thruster in terms of efficient plasma generation, gas heating, and thus thrust performance as well as system compactness; moreover, in f = 11 GHz discharges with longer Lₛ ≈ 3λ/4, standing-wave striation-like plasma structures occur in the axial direction. Experiments were made for f = 11 and 4 GHz microwaves with the respective Lₛ ≈ λ/4, where the plasma electron density and gas temperature in the microplasma source were measured by optical emission spectroscopy with a small amount of H₂ and N₂ added. The electron density and gas temperature were in the range of (4–12) × 10¹⁹ m⁻³ and 800–1000 K for f = 11 GHz, being ∼10%–50% higher than those for f = 4 GHz. The thrust performance was also measured by a target-type microthrust stand, giving a thrust, specific impulse, and thrust efficiency in the range 0.2–1.8 mN, 65–90 s, and 2%–14% for f = 11 GHz, which were ∼10%–15% higher than those for f = 4 GHz. These experimental results were consistent with those of simulations, depending on microwave frequency, power, chamber size, and gas flow rate.