A modified pseudo-equilibrium model competing with kinetic models to determine the composition of a two-temperature SF6 atmosphere plasma

This paper is devoted to calculation of the non-equilibrium composition in a SF6 thermal plasma at atmospheric pressure. Non-equilibrium thermal plasm as are characterized by heavy species temperatures T-h below 9000 K with el ectron temperatures at the maximum three times higher than Th when the latt er is below 4000 K. Different theories have been used based on either multi -temperature plasmas, Saha-Potapov modified by Andre et al, van de Sanden e t al, Cliteur et al, or kinetic calculations or the pseudo-equilibrium mode l, recently developed. This model gives results similar to those of kinetic calculations for N-2 and H-2 plasmas but with calculation times two orders magnitude faster. Pseudo-equilibrium. calculation takes into account the r eactions with low activation energies instead of ionization reactions, whil e keeping all the species present in the kinetic calculation. First, the th eories are compared in a case already studied in the literature by Cliteur: a heavy species temperature T-h, at 6000 K, with the electron temperature T-e varying between 6000 and 15 000 K. Comparison of the results shows that the multi-temperature calculations, except those of Cliteur, are far from kinetic especially for n(e) and n(F-). In addition, the pseudo-equilibrium model fits rather well with the kinetic calculations as long as molecular s pecies are present in the plasma. Second, to calculate the composition of n on-equilibrium thermal plasmas the ratio T-e/T-h is assumed to vary as the logarithm of the electron densities ratio n(e)/n(c)(max), n(e)(max) being t he electron density over which equilibrium prevails, i.e. 10(23) m(-3). For kinetic reactions where electrons are involved (in the direct reaction whi le heavy species intervene in the reverse reaction), a temperature T* betwe en T-e and T-h is defined. T* is calculated as a function of the electron f lux to that of heavy species. The variation of T* with Th is smoother than that of T-e. In such conditions again, there is an excellent agreement betw een kinetic and pseudo-equilibrium calculations performed at T*, which is n ot the case for multi-temperature calculations. These results demonstrate t hat the pseudo-equilibrium calculation developed for thermal plasma simple forming gases such as N-2 and H-2 can also be applied to more complex gases such as SF6.