3-BODY RECOMBINATION RATE OF ATOMIC NITROGEN IN LOW-PRESSURE PLASMA FLOWS

A nonlinear collisional-radiative model for atomic nitrogen has been d eveloped in order to determine effective recombination and associated ionization coefficients for either optically thin or optically thick p lasmas 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 equ al to 12 000 K. A time-dependent approach confirms the importance of t he quasi-steady-state condition to derive meaningful rate coefficients . In order to implement these coefficients in plasma flow codes, simpl e analytical expressions are proposed. Significant discrepancies with existing literature values are pointed out and discussed. In recombini ng conditions, the relaxation time necessary to reach the quasi-steady -state is calculated, and depends on the electron number density and o n the electron temperature. This time limits the validity of the recom bination coefficient, especially in optically thick cases. Finally, fo r practical applications, the number of levels in Saha-Boltzmann equil ibrium at the quasisteady-state is also examined.