mardi 2 juillet 2013 à 17:00
Results for the recombination rate in ionic plasmas are presented which are obtained by data processing of the experimental study of the gas discharge afterglow in fluorine and sulfur hexafluoride. The measurements point to the suppression of the recombination rate in those media with respect to the values which are predicted by the conventional formula for ideal plasmas. The slowdown degree increases with the increase of the plasma non-ideality parameter Γ. Theoretical analysis shows a number of specifics of the recombination in ionic plasmas in comparison to electron-ion plasmas. The specifics are caused by that the number density of neutrals is remarkably larger than that of ions in ionic plasmas
I) Modeling and simulation of the recombination in non-ideal ionic plasmas is performed similar to electron-ion plasmas. Numerical result are approximated by the expression
where k0 is the recombination rate for ideal plasmas, β is an adjustable parameter to fit the continuity in the point Г0 where the recombination mechanism changes. The expression differs from the Lankin formula for electron-ion plasmas by appearance of a new value Δ and by a decrease of the parameter A. Both differences are due to ion-neutral interactions which are treated in a twofold way. Weak long-range interactions are taken into account in the second virial approximation which results in the extension by Δ of the zone of the multi-particle fluctuations which separates the ranges of free and pair states; the value of Г0 decreases correspondingly. Strong short-range interactions are included in the simulation itself by means of additional stochastic forces acting on ions. It results in a certain lowering of A. The formula obtained compared with the measurements for the sulfur hexafluoride afterglow.
II) The recombination is suppressed by the solvation of ions in the weakly non-ideal fluorine ion plasmas. The process passes through a formation of an intermediate pair of cluster ions. The recombination rate approximate similarities are explained with respect to both temperature and pressure, which are observed in experiments. The rate dependence on Γ at small values of Γ is due to the dependence of equilibrium number density of loose cluster pairs on the ion fugacity, which is determined by Γ. The agreement with the measurements is found for the fluorine ion plasmas.
The formula is suggested for the recombination rate in ion plasmas which incorporates both plasma non-ideality and ions solvation. No adjustable parameter is used.