Implications of thermo-chemical instability on the contracted modes in CO2 microwave plasmas

Understanding and controlling contraction phenomena of plasmas in reactive flows is essential to optimize the discharge parameters for plasma processing applications such as fuel reforming and gas conversion. In this work, we describe the characteristic discharge modes in a CO2 microwave plasma and assess the impact of wave coupling and thermal reactivity on the contraction dynamics. The plasma shape and gas temperature are obtained from the emission profile and the Doppler broadening of the 777 nm O(S-5 <- P-5) oxygen triplet, respectively. Based on these observations, three distinct discharge modes are identified in the pressure range of 10 mbar to atmospheric pressure. We find that discharge contraction is suppressed by an absorption cutoff of the microwave field at the critical electron density, resulting in a homogeneous discharge mode below the critical transition pressure of 85 mbar. Further increase in the pressure leads to two contracted discharge modes, one emerging at a temperature of 3000 K-4000 K and one at a temperature of 6000 K-7000 K, which correspond to the thermal dissociation thresholds of CO2 and CO, respectively. The transition dynamics are explained by a thermo-chemical instability, which arises from the coupling of the thermal-ionization instability to heat transfer resulting from thermally driven endothermic CO2 dissociation reactions. These results highlight the impact of thermal chemistry on the contraction dynamics of reactive molecular plasmas.