Thermo-chemical dynamics and chemical quasi-equilibrium of plasmas in thermal non-equilibrium

We examine both processes of ionization by electron and heavy-particle impact in spatially uniform plasmas at rest in the absence of external forces. A singular perturbation analysis is used to study the following physical scenario, in which thermal relaxation becomes much slower than chemical reactions. First, electron-impact ionization is investigated. The dynamics of the system rapidly becomes close to a slow dynamics manifold that allows for defining a unique chemical quasi-equilibrium for two-temperature plasmas and proving that the second law of thermodynamics is satisfied. Then, all ionization reactions are taken into account simultaneously, leading to a surprising conclusion: the inner layer for short time scale (or time boundary layer) directly leads to thermal equilibrium. Global thermo-chemical equilibrium is reached within a short time scale, involving only chemical reactions, even if thermal relaxation through elastic collisions is assumed to be slow.