Radiative transfer in a hot plasma

The radiative heat flux in a hot (1 Mev), optically thick plasma (i.e. one where departure from equilibrium owing to a temperature gradient is small) depends on the opacity of the medium, the Rosseland mean opacity which is defined by an appropriate averaging of the photon mean path l(omega) over frequency. Small departures from equilibrium give rise to a photon momentum distribution which is assumed to be the Planck function corrected by a small quantity which depends on l(omega) (diffusion approximation) and positrons and electrons are assumed in thermal balance. It is then possible to linearize the relativistic Boltzmann transport equation for the photons, which yields a linear integral equation for l(omega). The relativistic Boltzmann transport equation for the photons involves microscopic processes of interactions for the radiation. For T around 1 Mev, two dominant processes have to be considered, Compton scattering and annihilation-creation of pairs.