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, T e , is seen to be in good agreement with the experimental measurements when spatial gradients of T e 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, p g , the phenomenon of diffusion cooling occurs in which T e relaxes to an equilibrium temperature, T ee , which is less than the gas temperature, T g . This reduction in T ee below T g is mirrored in a reduction in the ambipolar diffusion coefficient, D a , for the rare gas atomic ions and electrons. Thus the D a can be calculated as a function of p g using the values of T ee , and when this is done, properly accounting for the heating by metastable atoms, the calculated and experimental values of D a in all three rare gas afterglows are seen to be in agreement.