Relaxation Processes in a Non-Equilibrium Plasma

The Boltzmann kinetic equation method is used to describe non-steady-state processes in a non-equilibrium plasma. A plasma with local collisions is considered, in which the concentration of particles at a point is determined by the collision processes at that point and by the radiation field. Variations in the plasma parameters with time can be calculated by means of equations of particle and energy balance if the symmetrical part of the electron velocity distribution function is known. The author proposes a method for solving the system of equations of particle and energy balance. In the case of non-steady-state processes, quasi-stationary values can often be used for the photon frequency distribution and electron velocity distribution functions. Moreover, it can be assumed that the distribution of excited atoms by states is also quasi-stationary in character. In a plasma consisting of a mixture of an inert gas and alkali metal vapours capable of being used in MHD generators, the situation becomes even simpler. For virtually all regimes of interest from the point of view of MHD conversion, non-steady-state processes are quasi-stationary in character, i.e. the concentrations of electrons and atoms in all states of the discrete energy spectrum become adjusted to the changing temperature. This is due to the fairly uniform arrangement of levels in the term diagram of the alkali metal atoms and to the high intensity of bound-bound transitions. If the electron concentration is greater than 1013 cm-3 and the geometric dimensions greater than 1 cm, the distribution of excited atoms by states and the degree of ionization are in equilibrium with the electron temperature when more than 1% of the seed is ionized. Simple formulae are derived for calculating the concentrations and temperatures of the plasma components as functions of time. When it is possible to neglect variations in the temperature of the heavy particles, the equations are solved by quadrature. Simple criteria are obtained for the quasi-stationary state, for thermodynamic equilibrium, and for the presence of local collisions; these criteria take into account non-uniformity of the irradiation field and deviations of the electron velocity distribution functions from the equilibrium value. If any of these criteria is not fulfilled it is necessary to use the general theory of non-steady-state processes in a non-equilibrium plasma, previously developed by the author. (author)