This work addresses plasma chemistry in the core of a vortex-stabilized microwave discharge for CO2 conversion numerically, focusing on the pressure-dependent contraction dynamics of this plasma. A zero-dimensional model is presented for experimental conditions in a pressure range between 60 and 300 mbar and a temperature range between 3000 and 6500 K. Monte Carlo flux (MCF) simulations, which describe electron kinetics, are self-consistently coupled to the plasma chemistry model. The simulation results show that an increase in pressure is accompanied by a transition in neutral composition in the plasma core: from a significant amount of CO2 and O2 at low pressures to a O/CO/C mixture at high pressures, the composition being determined mostly by thermal equilibrium and by transport processes. The change of temperature and composition with pressure lead to higher ionisation coefficient and more atomic ion composition in the plasma core. These changes result in an increase in ionisation degree in the plasma core from 10−5 to 10−4. These factors are shown to be fundamental to drive contraction in the CO2 microwave discharge.

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Thermalization of electron and gas temperature in CO2 microwave plasma is unveiled with first Thomson scattering measurements. The results contradict the prevalent picture of an increasing electron temperature that causes discharge contraction. It is known that as pressure increases, the radial extension of the plasma reduces from ~7 mm diameter at 100 mbar to ~2 mm at 400 mbar. We find that, simultaneously, the initial non-equilibrium between ~2 eV electron and ~0.5 eV gas temperature reduces until thermalization occurs at 0.6 eV. 1D fluid modelling, with excellent agreement with measurements, demonstrates that associative ionization of radicals, a mechanism previously proposed for air plasma, causes the thermalization. In effect, heavy particle and heat transport and thermal chemistry govern electron dynamics, a conclusion that provides a basis for ab initio prediction of power concentration in plasma reactors.

This work investigates kinetics and transport of CO2 microwave plasmas through simulation results from a 1-D radial fluid model and experiments. Simulation results are validated against spatially resolved measurements of neutral species mole fractions, gas temperature, electron number density and temperature obtained by means of Thomson and Raman scattering diagnostics, yielding good agreement. As such, the model is used to complement experiments and assess the main chemical reactions, mass and energy transport in diffuse and contracted plasma regimes. From model results, it is found that, as pressure is raised, the inhomogeneous gas heating induces significant gradients in neutral and charged species mole fractions profiles. Moreover, the transition from diffuse to contracted plasma is accompanied by a change in the dominant charged species, which favours electron-ion recombination over dissociative attachment. Associative ionization rates increase in the plasma core from diffuse to contracted regime. These processes contribute to the increase in the peak electron number density with pressure, that determines radial plasma contraction.