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traveled is referred to as the radiation mean free path, and the transport of energy by radiation in the opaque medium approximates the kinetic theory process of diffusion (61).

        In the presence of radiation, an atom or ion has a certain probability of either absorbing or emitting radiation (induced transitions) of a frequency characteristic of some transition from one quantum state to another. An excited atom or ion can also spontaneously emit radiation having a characteristic frequency. Each atom or ion therefore has an absorption spectrum which can be expressed in terms of an absorption coefficient, K 1, which is frequency dependent. Since the absorption spectrum depends upon the distribution of plasma species in their various quantum states, K depends on the plasma temperature and the particle density. As the plasma density increases, pressure broadening changes the emission spectrum (61), so that K also depends on the density through this effect. The frequency dependent absorption coefficient for photons can be expressed as a function of temperature and density as follows (62):



        K (T,p)=    N (T,P)ab() (i - ehv/kT )in cm
                  a