IONIZATION AND RECOMBINATION RATES OF ATOMIC OXYGEN IN HIGH-TEMPERATURE AIR PLASMA FLOWS

A nonlinear time-dependent collisional-radiative model for atomic oxyg en is presented. Effective ionization and three-body recombination rat e coefficients are calculated for either optically thin or thick plasm as where 10(18) m(-3)less than or equal to n(e) less than or equal to 10(21) m(-3) and 8000 K less than or equal to T-e less than or equal t o 20 000 K. In order to implement these coefficients easily in plasma flow codes, simple analytical expressions are proposed. Significant di screpancies with existing literature values are pointed out and discus sed. The time-dependent approach confirms the importance of a quasiste ady-state condition to derive meaningful rate coefficients. Therefore the relaxation time necessary for the system to reach a quasi-steady-s tate limits the validity of the rate coefficients determined in this s tudy. In optically thick cases, this relaxation time in either recombi ning or ionizing plasmas depends weakly on the electron temperature bu t strongly increases as the electron number density decreases. In opti cally 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.