Electron Temperature Relaxation in Afterglow Plasmas: Diffusion Cooling

We have carried out a thorough theoretical analysis of the cooling and heating processes of the electron gas in Ne, Ar and Kr afterglow plasmas. Thus the rate of relaxation of the electron temperature, Te, is seen to be in good agreement with the experimental measurements when spatial gradients of Te in the early afterglow and heating of the electron gas by superelastic collisions between the electrons and metastable atoms are accounted for. At low pressures of the rare gases, pg, the phenomenon of diffusion cooling occurs in which Te relaxes to an equilibrium temperature, Tee, which is less than the gas temperature, Tg. This reduction in Tee below Tg is mirrored in a reduction in the ambipolar diffusion coefficient, Da, for the rare gas atomic ions and electrons. Thus the Da can be calculated as a function of pg using the values of Tee, and when this is done, properly accounting for the heating by metastable atoms, the calculated and experimental values of Da in all three rare gas afterglows are seen to be in agreement.