Criteria for local thermal equilibrium in non-hydrogenic plasma

Rate equations for collisional processes are formulated and used to obtain the minimum electron density necessary to ensure LTE of a particular atomic species, with particular attention to the existance of low-lying metastable levels. The final expression is compared with an earlier criterion of Griem. The results of various experiments on plasmas containing one or more ionization stages of helium, nitrogen, oxygen, and argon are discussed. The importance of making reliable estimates of collision strengths, for excitation involving metastable levels, is emphasized. (author)