Three-body recombination rate of atomic nitrogen in low-pressure plasma flows

A nonlinear collisional-radiative model for atomic nitrogen has been developed in order to determine effective recombination and associated ionization coefficients for either optically thin or optically thick plasmas where 10(18) m(-3)less than or equal to n(e) less than or equal to 10(21) m(-3) and 2000 K less than or equal to T-e less than or equal to 12 000 K. A time-dependent approach confirms the importance of the quasi-steady-state condition to derive meaningful rate coefficients. In order to implement these coefficients in plasma flow codes, simple analytical expressions are proposed. Significant discrepancies with existing literature values are pointed out and discussed. In recombining conditions, the relaxation time necessary to reach the quasi-steady-state is calculated, and depends on the electron number density and on the electron temperature. This time limits the validity of the recombination coefficient, especially in optically thick cases. Finally, for practical applications, the number of levels in Saha-Boltzmann equilibrium at the quasisteady-state is also examined.