Criteria for the existence of local thermal equilibrium in various classes of plasmas are derived. In addition to general criteria that should also be fulfilled for homogeneous and time-independent plasmas, special criteria must be imposed on inhomogeneous and/or transient plasmas. The influence of the imprisonment of resonance radiation on the establishment of thermal equilibrium is considered, and it is concluded that even then complete thermal equilibrium can usually not be achieved for multiply ionized species.

• Received 18 March 1963

DOI:https://doi.org/10.1103/PhysRev.131.1170