Influence of water vapor concentration on discharge dynamics and reaction products of underwater discharge within a He/H_2O-filled bubble at atmospheric pressure

In this study, a two-dimensional fluid model is proposed to simulate the underwater discharge ina He/H₂O-filled bubble at atmospheric pressure. The molar fraction of water vapor is varied inthe range of 0.01%–1% to investigate the dependence of discharge dynamics and reactionproducts on water vapor concentration (WVC). The numerical results show that most propertiesof the discharge sensitively depend on the WVC. The increase of WVC leads to an increase inthe electron density and discharge propagation velocity, which is attributed to Penning ionizationbetween He* and H₂O. The main positive ion switches from He⁺ to H₂O⁺, while the WVCincreases from 0.01% to 1%. The dominant reactive oxygen species is OH, whose peak densityis about two orders of magnitude higher than that of O. Besides, the densities of OH and Oradicals increase with the increasing WVC. It is shown that the formation mechanism of Oradicals is significantly affected by the WVC. The dominant reaction creating O radicals changesfrom the charge exchange between He₂⁺ and H₂O to the electron impact dissociation of H₂O asthe WVC increases from 0.01% to 1%. This study is helpful for better understanding theapplication of non-thermal plasmas discharges in water, such as biomedical, environmentalengineering.