Hybrid global model of water cluster ions in atmospheric pressure Ar/ H₂O RF capacitive discharges

Water is a trace gas of strong interest for plasma-based medical applications. We use a hybrid global model to simulate a chemically complex Ar/${\text{H}}_2\text{O}$ atmospheric pressure, radio frequency capacitive discharge, including 47 species with positive ion clusters up to ${\text{H}}_{21}{\text{O}}_{10}^{+}$. For a discharge gap of 1.5 mm driven at 27.12 MHz, we determine the discharge properties over a range of rf currents (150–500 A m⁻²) and initial ${\text{H}}_2\text{O}$ concentrations (0.25–2%). An isothermal plug-flow model is used with a gas residence time of 0.2 s for most calculations, with the gas temperature calculated self-consistently from the input power. The cluster density distributions are determined, and we find that the higher mass cluster densities decrease rapidly with increasing gas temperature. A simplified cluster dynamics analytic model is developed and solved to determine the cluster density distributions, which is in good agreement with the hybrid simulation results. (paper)