Plasma simulations

Abstract

The complete thermodynamical equilibrium is an ideal case. Deviations from this ideal state appear when the radiation energy distribution departs from Planck’s law. However, in many cases, one can consider that the matter still remains locally in thermodynamical equilibrium at some temperature. This is the so-called LTE equilibrium, discussed in Chap. 6. Many plasma simulations where the radiation transfer is coupled to hydrodynamics are performed in this framework. The justification is that the spatial zones where radiation energy transfer is important have often a density which is high enough to warrant the validity of LTE conditions.

Independently of this possible coupling between matter and radiation, there exist many situations where matter deviates strongly from LTE. When the variation of the plasma parameters is faster than the time scales of ionization and recombination, one has to resort to a time-dependent population kinetics modeling (otherwise one deals with steady-state population kinetics). Some practical aspects of these calculations are discussed.

The former situation (matter at LTE), where it is important to consider the influence of radiation on the hydrodynamical evolution of a plasma, corresponds to the so-called radiation energy transfer problem. Its treatment relies on various approximations are reviewed. In particular, this leads to the definition of appropriate average opacities such as the Rosseland opacity, derived in the diffusion approximation of the radiation transport equation.