A Capacitively Coupled Discharge Operating on Pure Water Vapor and Mixture with Helium at Low and Close to Atmospheric Pressure

Abstract

A one-dimensional fluid model has been developed and used to investigate a capacitively coupled discharge operating on a pure water vapor and on a mixture of water with helium. These models incorporate 29 main species and 81 dominant reaction channels. The effects of gas pressure and gap distance on the production of some species are discussed. The simulation results reveal that the plasma ignition in a pure water vapor can only be reached at low pressure when all dominant reactions are included in the model. The main mechanism by which the plasma ignition is suppressed may be attributed to the high rate of electron cooling. It is worth indicating that the creation rates of the vibrationally and rotationally excited states in water vapor compared to ionization are much more higher than they are in noble gases, such as argon or helium. As a result, this might be the main reason why the ignition of He plasma is easier when applying the same operating parameters. Therefore, two approaches are suggested to overcome this difficulty. The first one lies in decreasing the gas pressure to around \(10^3\) Pa, and the gap distance to 0.5 mm. And the second one is by adding a small fraction (5%) of \(\mathrm{H}_2\mathrm{O}\) to (95%) of helium and keeping the gap distance at 1 mm. The input parameters for plasma ignition have been identified, and the density profiles of the main species are reported and discussed.