Objective With the rapid development of aerospace engineering and the increasing demand for high-performance microsatellites, microwave plasma thrusters (MPT) have emerged as one of the most promising electric propulsion technologies for orbit maintenance, attitude control, station keeping, and deep-space exploration missions. Featuring high specific impulse, extremely low plume contamination, compact modular structure, and excellent long-term operational reliability, MPT offers significant advantages over traditional chemical propulsion systems, particularly in extending mission duration and reducing launch mass. However, the performance of MPT is fundamentally determined by the internal plasma characteristics, and the accurate, real-time, and non-intrusive diagnosis of key plasma parameters—most critically gas temperature—has long been a major technical bottleneck restricting further performance optimization and practical engineering application. Conventional diagnostic techniques such as Langmuir probes inevitably disturb the delicate plasma structure and suffer from severe probe tip ablation under high-temperature conditions, while laser-induced breakdown spectroscopy (LIBS) requires complex and expensive optical systems, making it impractical for continuous, full-domain monitoring inside the thruster resonant cavity. Therefore, developing a reliable, low-cost, and non-intrusive plasma parameter diagnostic method is of paramount importance for advancing MPT technology from laboratory research to industrial deployment.

Methods Helium (He), an inert monatomic gas with a high ionization energy of 24.59 eV, abundant and well-separated characteristic spectral lines in the visible and near-infrared regions, and exceptional chemical stability, is selected as the ideal diagnostic propellant in this study. It effectively overcomes the inherent limitations of other commonly used working fluids: water vapor undergoes significant dissociation at high temperatures, producing complex molecular spectra that overlap with plasma emission lines; ammonia (NH₃) exhibits severe spectral interference due to its complex molecular structure; and hydrogen (H₂) has very few usable spectral lines within the detection range of most commercial spectrometers. In this work, a comprehensive non-intrusive parameter diagnosis system based on He atomic emission spectroscopy is established to achieve precise quantitative measurement of gas temperature. The experimental platform consists of a 2.45 GHz magnetron with continuously adjustable output power, a cylindrical resonant cavity designed to operate in the TM₀₁₁ mode, a high-precision mass flow controller, and a vacuum system maintained at approximately 3 Pa during all experiments. A NOVA-EX fiber-optic spectrometer equipped with thermoelectric cooling technology is employed to capture plasma emission spectra, with a detection range of 319–1106 nm, a slit width of 25 μm, a wavelength resolution of 0.76 nm, and an optical resolution of 1.97 nm. Five distinct He mass flow rates (100, 200, 300, 400, and 500 sccm) are tested at a constant microwave input power of 1100 W, and the Boltzmann plot method is applied to calculate gas temperature using four well-characterized He atomic transition lines (388.9 nm, 587.6 nm, 667.8 nm, and 706.5 nm). The Hα line at 656.3 nm, originating from minor air leakage into the cavity, is carefully excluded from the analysis. The local thermodynamic equilibrium (LTE) condition is rigorously validated by the high linearity of the Boltzmann plots, with all fitting R² values exceeding 0.85 and a maximum value of 0.974 achieved at 400 sccm.

Results and Discussions Key experimental results demonstrate that the He mass flow rate exhibits a strong synchronous negative correlation with both the resonant cavity gas temperature and the thruster specific impulse. In the range of 100–400 sccm, both parameters decrease relatively gently due to stable microwave-plasma coupling efficiency and moderate energy transfer between excited and ground-state particles. However, when the flow rate exceeds 400 sccm, the decline rate increases dramatically. This abrupt change is attributed to three interconnected physical mechanisms: first, the specific energy (defined as the ratio of input microwave power to propellant mass flow rate) decreases significantly; second, excessive plasma density causes the plasma frequency to exceed the microwave frequency, leading to enhanced microwave shielding and skin effects that drastically reduce energy absorption efficiency; and third, increased particle collision frequency accelerates energy dissipation through inelastic collisions and thermal radiation. Under the LTE approximation, specific impulse is directly proportional to the square root of gas temperature, as temperature determines the average thermal kinetic energy of propellant particles and thus their exhaust velocity after expansion through the Laval nozzle. Furthermore, while variations in microwave input power do not alter the fundamental negative correlation between flow rate and performance parameters, they significantly affect the absolute values of temperature and specific impulse, as well as the critical threshold for abrupt performance degradation. Higher input power increases the specific energy per unit mass, shifting the critical threshold toward higher flow rates, whereas lower power reduces overall performance and shifts the threshold leftward. Extremely low power may even break the LTE condition, weakening the linear correlation in Boltzmann plots and reducing diagnostic accuracy. The measured gas temperature and specific impulse values show excellent agreement with previously published computational and experimental results, confirming the reliability and accuracy of the proposed diagnostic system.

Conclusions This study provides a robust, non-intrusive method for high-precision parameter diagnosis of microwave plasma thrusters, reveals the intrinsic physical mechanisms governing the effects of He flow rate and microwave power on gas temperature and specific impulse, and clarifies the complex energy coupling processes between microwave fields and plasma. The findings offer valuable experimental data and theoretical support for propellant flow optimization, resonant cavity structural design improvement, microwave coupling efficiency enhancement, and numerical simulation validation of MPT. For practical engineering applications, low flow rates (100–300 sccm) are recommended for high-specific-impulse missions such as deep-space exploration, while the flow rate should be strictly controlled below 400 sccm to maintain stable and efficient propulsion performance. This work advances the development of accurate diagnostic technologies for microwave plasma thrusters and lays a solid foundation for their widespread engineering application in future aerospace propulsion systems.