Fluid equations for a partially ionized plasma

Transport coefficients and fluid equations in the limit of short mean free path are derived for a partially ionized plasma undergoing charge exchange, ionization, and recombination. The electron distribution is assumed to be unaffected by these processes and by collisions with neutral particles, and collisions among the neutrals are neglected. The calculated heat flux and viscosity contain terms purely related to charge exchange, and terms proportional to their usual counterparts in a completely ionized plasma with an additional proportionality factor equal to the inverse square of the fraction of ions in the plasma. The reason for this latter behavior is that charge exchange does not randomize particle trajectories, so that heavy particles suffer an ‘‘identity crisis’’ by existing alternately as neutrals and ions. The use of a closer approximation to the charge-exchange cross section than previously employed gives rise to new terms in the neutral momentum and energy equations, representing the thermal force and the diffusion thermoeffect. The latter contributes significantly to the total heat flux in the plasma.