Microwave-driven Plasma for Low-power Microsatellite Propulsion

Microwave-driven Coaxial Transmission Line Resonator (μ-CTLR) is a compact plasma source operable for a wide pressure range—from atmospheric pressure down to a few mTorr—owing to the high electric field intensity at the open end of its resonant electrode. Under low-pressure conditions, the μ-CTLR generates a small-volume, high-density plasma plume (~10²⁰m⁻³) with less than 10 W input power, making it a promising candidate for miniature space propulsion systems that demand compact size, stable operation, and low power consumption [1]. In our previous study, a μ-CTLR operating at 900 MHz and 8 W with an argon mass flow rate of 100 SCCM (3 mg/s) showed a thrust of 3.4 mN, specific impulse of 116 s, total efficiency of 24 %, and a thrust-to-power ratio of 425 mN/kW [1]. In this study, we investigate the performance of a μ-CTLR integrated with a magnetic nozzle (MN) composed of annular permanent magnets to enhance specific impulse and efficiency. The system operates stably with argon gas flow rates below 2.5 SCCM (0.075 mg/s), producing a visibly expanding plasma plume under vacuum conditions (operating pressure ~ 10^-4 Torr; base pressure ~10^-7 Torr). The MN configuration provides a maximum axial magnetic field strength of approximately 200 G near the nozzle throat, contributing to plasma confinement and acceleration. Near the open end of the electrode, the electron density reaches ~5 × 10¹⁹ m^-3, increasing significantly as the mass flow rate decreases. Moreover, ion energy measurements using a Retarding Potential Analyzer (RPA) indicate fast ion populations with energies up to ~50 eV. Our results propose the μ-CTLR with a magnetic nozzle as a highly efficient mN-class thruster for miniature satellites, with performance comparable to or exceeding other plasma thrusters [2–3].

[1] Kim K. et al., Plasma Sources Sci. Tech., 33(8), 085003, (2024)

[2] Takahashi T., et al., Journal of Applied Physics, 125(8), 083301, (2019)

[3] Zolotukhin, D. B., et al., Science Advances, 8(36), eadc9850, (2022)