Thermal equilibrium criteria in a nitrogen plasma

A method for obtaining the lower electron density limit for LTE in a nitrogen plasma is described, whereby the experimentally determined ratio of the collisional-radiative ionization and recombination coefficients (S/α) is compared with the corresponding LTE value (Saha ratio). It is argued that if the electron density is increased from values of about 10¹⁶cm^-3, S/α should tend to the Saha ratio as LTE is approached. For NII and NIII spectral lines, this is found to happen at an electron density of a few times 10¹⁶cm^-3 when the electron temperature is about 3 eV, in good agreement with the LTE criterion of Griem.