Effect of water vapor on plasma morphology, OH and H₂O₂ production in He and Ar atmospheric pressure dielectric barrier discharges

Although atmospheric pressure dielectric barrier discharges (DBDs) have a long history, the effects of water vapor on the discharge morphology and kinetics have not been studied intensively. We report a simultaneous investigation of discharge morphology, OH and H₂O₂ production in Ar and He DBDs operated at different water vapor concentrations and powers. The combined study allows us to assess the impact of the discharge morphology and power on the concentration dependence of the OH and H₂O₂ production. The morphology of the discharge is investigated by ICCD images and current-voltage waveforms. These diagnostics are complemented by broadband absorption and a colorimetric method to measure the gas temperature and the OH and H₂O₂ concentrations. The number of filaments in Ar DBD increases with increasing water concentration and power. The surface discharge part of the micro-discharge also reduces with increasing water concentration most likely due to a change in surface conductivity of the dielectric with changing water concentration. The OH density in the case of Ar is approximately double the OH density in He for similar power and water admixture. In contrast to the root square dependence of the OH density on the water concentration in He similar to diffuse RF discharges, the OH density in Ar increases for small water concentrations followed by a saturation and reduces for higher water concentrations. This dependence of OH density on water concentration is found to correlate with changes in discharge morphology. An analytical balance of the production and destruction mechanism of H₂O₂ is shown to be able to reproduce the ratio of the measured OH and H₂O₂ density for realistic values of electron densities.