O at atmospheric pressure*

The effects of the humidity of the feed gas on the discharge chemistry need to be considered to control the plasma chemistry. Detailed studies are scarce and often dominated by surface interactions, obscuring any volume effects. Here, volume kinetics are studied in a negative nanosecond pulsed discharge generated in a pin–pin 3 mm gap geometry in He + H₂O. The effect of humidity on the discharge development, electric field and electron density is investigated through experiments and modelling. It is found that the presence of water vapour affects both the electron density at the start of the pulse (remaining from the previous pulse) and the ionisation rates during the ignition phase, leading to a complex dependence of the discharge development speed depending on the water concentration. The electron decay is studied using the 0D global kinetics model GlobalKin. The dominant reactions responsible for the electron decay are determined by comparing experimental and simulated results and these reactions are grouped in simplified kinetic models. It is found that with water concentrations increasing from 0 to 2500 ppm, the complexity of the dominant reactions increases with in particular O₂ + H₂O₃ and water clusters becoming important for high water concentrations.