Weak nonequilibrium states of photons in equilibrium plasma

The photon kinetics in equilibrium plasma medium is investigated on the basis of the Bogolyubov reduced description method. A starting point of the analysis is a linearized kinetic equation which takes into account the Compton and the bremsstrahlung photon processes. The photon polarization, electron spin phenomena and plasma ion effects are neglected. It is assumed that all processes in the system can be considered in quasi-relativistic approximation. In this approximation change of a photon (an electron) energy in the Compton process is small that is described by small parameter λ. The states of the system are considered as spatially weakly non-uniform that is described by small parameter g. A situation is investigated in which contribution of the bremsstrahlung processes is small compared with the Compton processes (for simplicity this is described by the small parameter g too). Reduced description of photons in the plasma by photon energy distribution is investigated. The consideration is based on the Bogolyubov idea of the functional hypothesis that leads to a generalization of the Chapman-Enskog method. The kinetic equation for the photon energy distribution is obtained that describes the photon diffusion similar to the known neutron diffusion equation.