Influence of propellant on radio-frequency discharge modes transition characteristics

Radio-frequency plasma thrusters, as a type of electrode-less electric thruster, represent a preferred thruster solution for the long-duration operation of air-breathing electric propulsion systems in very low Earth orbits. However, ambient parameters significantly influence the radio-frequency discharge modes and the transition characteristics, which directly determine the thruster's operational stability under a wide range of conditions. We employed test diagnostic apparatus such as Langmuir probes and optical emission spectroscopy to measure the variations of plasma density and spectral intensity with radio-frequency power under different working conditions, including propellant type, mixture ratio, and flow rate. The influence of the propellant on the transition characteristics of radio-frequency discharge modes was subsequently analyzed. For argon, the discharge mode transition from capacitively coupled plasma to inductively coupled plasma occurs at 300 W, accompanied by consistent jumps in plasma density and spectral intensity. Due to its higher ionization energy, the required radio-frequency power threshold for discharge mode transition is significantly higher for nitrogen than for oxygen. In nitrogen-oxygen mixtures, an increased oxygen proportion reduces the transition threshold; however, a high oxygen fraction can facilitate the formation of negative ions due to its electronegativity, thereby suppressing the free electron density. The threshold of radio-frequency power for mode transition increases with the increase of gas flow rate, driven by the reduced gas residence time, the heightened electron-neutral collision frequency, and the consequent decline in ionization efficiency. The results provide important test evidence for elucidating the characteristics of radio-frequency plasma discharge mode transitions and for optimizing the operational parameters of radio-frequency plasma thrusters under wide-range conditions.