A theoretical insight into low-temperature atmospheric-pressure He+H2 plasmas

Abstract

H-containing low-temperature plasmas are used in a wide range of industrial applications. In recent decades, efforts have been made to understand and improve the performance of these plasmas, mainly when operated at low and medium pressures. Studies of hydrogen-containing plasmas at atmospheric pressure, however, are scarce despite the potential advantage of operation in a vacuum-free environment. Here the chemistry of low-temperature atmospheric-pressure He + H plasmas is studied by means of a global model that incorporates 20 species and 168 reactions. It is found that for a fixed average input power the plasma density decreases sharply when the H concentration is higher than ∼0.2%, whereas the atomic H density peaks at a H concentration of ∼2%. Operation at larger H concentrations leads to lower plasma densities and lower H concentrations because at high H concentrations significant power is dissipated via vibrational excitation of H and there is an increasing presence of negative ions (H). Key plasma species and chemical processes are identified and reduced sets of reactions that capture the main physicochemical processes of the discharge are proposed for use in computationally demanding models. The actual waveform of the input power is found to affect the average density of electrons, ions and metastables but it has little influence on the density of species requiring low energy for their formation, such as atomic hydrogen and vibrational states of hydrogen. © 2013 IOP Publishing Ltd.