Kinetic theory in self-similar variables for neutral atoms in plasma

The kinetics of neutral atoms in a plasma undergoing charge-exchange, ionization, and recombination is considered. It is shown that if the mean-free path divided by the macroscopic scale length is constant, it is possible to introduce self-similar variables in the Boltzmann equation for the neutral particles. This equation is then solved analytically, and the nonlocal transport of heat and particles is calculated. Since the mean-free path increases with increasing energy, a one-sided, high-energy tail is formed in the neutral distribution function. This tail may contribute significantly to the heat and particle fluxes. When this is the case, the fluid approximation of these quantities breaks down at arbitrarily short mean-free paths. copyright 1996 American Institute of Physics