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               B (T) = 2hv' (ehv/kT -    -          (8.3)
               V        c2


For the optically thick medium, the radiation flux, qr' is expressed as a function of temperature by the Rosseland diffusion approximation (64,65):


                    qr=    K RVT                    (8.4)

where KR is the diffusion coefficient given by KR - 16 aT3                                        (8.5)
                         R= KR

KR is the Rosseland mean opacity defined by (8.2) and a is the Stefan-Boltzmann constant.

        Equation (8.4) is the simple kinetic theory

expression for the diffusion of photons having a mean free path XR = (I/KR] cm.

        The energy balance for a unit volume within a

coaxially flowing plasma region at constant pressure can be expressed as (viscous dissipation neglected): DH                      S - V • q                  (8.6)


where

          DH   rate of enthalpy increase per unit volume
          Dt
          S =rate of heat generation per unit volume


V • q = energy loss rate from the unit volume