Theoretical insight into Ar–O2 surface-wave microwave discharges

Abstract

A zero-dimensional kinetic model has been developed to investigate the coupled electron and heavy-particle kinetics in Ar–O2 surface-wave microwave discharges generated in long cylindrical tubes, such as those launched with a surfatron or a surfaguide. The model has been validated by comparing the calculated electron temperature and species densities with experimental data available in the literature for different discharge conditions. Systematic studies have been carried out for a surface-wave discharge generated with 2.45 GHz field frequency in a 1 cm diameter quartz tube in Ar–O2 mixture at 0.5–3 Torr pressures, which are typical conditions found in different applications. The calculations have been performed for the critical electron density for surface-wave propagation, ne = 3.74 × 1011 cm−3.It has been found that the sustaining electric field decreases with Ar percentage in the mixture, while the electron kinetic temperature exhibits a minimum at about 80%Ar. The charged and neutral species densities have been calculated for different mixture compositions, from pure O2 to pure Ar, and their creation and destruction processes have been identified. The O2 dissociation degree increases with Ar addition into O2 and dissociation degrees as high as 60% can be achieved. Furthermore, it has been demonstrated that the dissociation degree increases with the discharge tube radius, but decreases with the atomic surface recombination of O-atoms. The density of O− negative ions is very high in the plasma, the electronegativity of the discharge can be higher than 1, depending on the discharge conditions.