Low-pressure RF-ICP CO2 conversion relevant to Mars ISRU: Pressure-dependent kinetics and pathway contributions

CO2 conversion was investigated in a 13.56 MHz radio frequency inductively coupled plasma (RF-ICP) operated at 5–40 Pa, with emphasis on electron kinetics and dissociation pathways. The effects of coil turns and applied power were systematically evaluated using V–I measurements, Langmuir probe diagnostics, and optical emission spectroscopy. The optimized three-turn planar coil achieved a peak power-transfer efficiency of 66.9% and a maximum CO2 conversion of 37.1%. Electron density exhibited a non-monotonic pressure dependence, peaking at approximately 10–15 Pa. At lower pressure, reduced collisionality limited electron-impact reaction rates, whereas at higher pressure, decreases in electron temperature and density suppressed electron-impact dissociation and ionization. A two-dimensional fluid-chemistry model reproduced the experimentally observed conversion trend and revealed a pressure-induced shift in the dominant dissociation pathway. The ionization–recombination pathway contributed strongly at ≤15 Pa, whereas direct electron-impact dissociation became dominant above 15 Pa as the ionization pathway weakened. These results establish an experimentally anchored framework for low-pressure RF-ICP CO2 conversion and provide a mechanistic basis for future extension and validation under higher-pressure conditions relevant to Mars ISRU.