Ionization and recombination rates of atomic oxygen in high-temperature air plasma flows

Abstract

A nonlinear time-dependent collisional-radiative model for atomic oxygen is presented. Effective ionization and three-body recombination rate coefficients are calculated for either optically thin or thick plasmas where ${10}^{18}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}<~{n}_{e}<~{10}^{21}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ and $8000\mathrm{K}<~{T}_{e}<~20000\mathrm{K}.$ In order to implement these coefficients easily in plasma flow codes, simple analytical expressions are proposed. Significant discrepancies with existing literature values are pointed out and discussed. The time-dependent approach confirms the importance of a quasi-steady-state condition to derive meaningful rate coefficients. Therefore the relaxation time necessary for the system to reach a quasi-steady-state limits the validity of the rate coefficients determined in this study. In optically thick cases, this relaxation time in either recombining or ionizing plasmas depends weakly on the electron temperature but strongly increases as the electron number density decreases. In optically thin cases, it is more difficult to derive general results since the relaxation time depends on the electron temperature, on densities, but also on the initial distribution on atomic levels.