Validity conditions for local thermodynamic equilibrium

Abstract

Validity criteria for partial and complete local thermodynamic equilibrium (L.T.E.) are re-examinated on the basis of generalized cross section formulas which are in agreement with experiments and with quantum mechanical results. The validity conditions for L.T.E. obtained deviate both in the absolute values and in the temperature dependence from those given by other authors. The influence of resonance absorption on the establishment of L.T.E. is considered and quantitatively taken into account. The influence of atom-atom collisions on the population densities of highly excited levels is also discussed. A general formula is derived permitting an estimate wether the excited levels are populated due to electronic or due to atomic collisions. The condition obtained is quite severe for plasmas in which the electron temperature is different from the gas temperature. For example: In a helium non-L.T.E. plasma of 1% ionization degree (forT _e=2×10⁴ °K) all levels of principal quantum numbersn>12 will essentially be populated due to atomic collisions at a gas temperature ofT _a=1.6×10⁴ °K. The influence of diffusion on the L.T.E. conditions is discussed for the various kinds of particle interactions. It is found that deviations from L.T.E. due to particle diffusion may be more severe than is generally assumed.