Reactive species in cold atmospheric-pressure He+Air plasmas: The influence of humidity

Abstract

A He+Air mixture is a common working gas for cold atmospheric-pressure plasmas which compromises the discharge stability of helium and the chemical reactivity of air. Water vapor is an important component in the gas mixture which can be either introduced as a precursor for reactive species or inevitably entrapped in the plasma system from the surroundings, and the presence of water can cause a profound change on the characteristics of the plasma and consequently its application efficacy. A global model of cold atmospheric-pressure He+Air plasmas is presented in this paper so as to study the influence of humidity. The densities of reactive species and power dissipation are studied as a function of water concentration spanning from 10 to 10 000 ppm. It is found that the dominant cation changes from NO+ to H3O+, and the dominant anion changes from NO2− to OH− with the increasing water concentration. HNO2 and O2(a) are the most abundant reactive nitrogen species and reactive oxygen species, respectively. When the water concentration is above 100 ppm, the electronegativity of the plasma increases dramatically, which has a significant effect on the way of power dissipation in the plasma. Also, the plasma chemistry under different air contents (100, 1000, and 10 000 ppm) is studied as a function of the water concentration. Among different cases of air concentration, the presence of water can enrich the density of most hydrogen-containing species (OH, H2O2, HO2, H3O+, and HNO3) except for HNO2, and it can also enrich the density of hydrogen-free species [O, O3, O2−, and O2(a)] at a low air concentration (100 ppm) while running it down at a high air concentration (1000 and 10 000 ppm).