Self-consistent electron energy distribution functions, vibrational distributions, electronic excited state kinetics in reacting microwave CO2 plasma: An advanced model

Abstract

An advanced model for the calculation of electron energy distribution functions (eedfs), vibrational distributions, and electronic excited state densities of reacting CO2 in microwave (MW) discharges has been developed for clarifying: (1) the role of electronic states of the relevant neutral species in affecting the eedf and (2) the contribution to the CO2 dissociation of the electron impact and heavy particle dissociation mechanisms. To model the discharge, the power density typical of MW discharges is used as a parameter. Different case studies including optically thick and thin plasmas and the dependence of the CO2 dissociation rates on the gas temperature are investigated. The results show that at a low gas temperature, i.e., 300 K, the heavy-particle dissociation mechanism, also called the pure vibrational mechanism, prevails on the electron impact dissociation one, while at a high gas temperature, i.e., 2000 K, the two mechanisms become competitive and the global behavior strongly depends on the choice of electron impact dissociation cross sections. Large differences appear in the eedf, especially in the post-discharge regime, when considering thick and thin plasmas. In the thick case, a well-structured eedf appears as a result of superelastic collisions mainly involving the electronic states of the relevant neutral species. In the thin plasma, many peaks disappear because the concentration of the excited states strongly decreases. Finally, our model gives the results of conversion and energy efficiency as well as vibrational distributions in satisfactory agreement with the corresponding results calculated by the Antwerp group.