COMPARATIVE STUDY BETWEEN KINETIC AND THERMODYNAMIC CALCULATION OF COMPOSITION IN SF₆ PLASMAS

For an SF₆ plasma at atmospheric pressure the compositions calculated in chemical equilibrium, in the multi-temperature model and using kinetics (33 reactions involving 18 species) are compared. The ratio of the electron temperature T_e to that of heavy species T_h was calculated as a function of electron density and an intermediate temperature T^* between T_e and T_h was introduced to cope with the relationship k_f(T^*) / k_r(T^*) = K_x(T^*) where k_f and k_r are the reaction rate coefficients (forward and reverse) and K_x the equilibrium constant. The main difference between equilibrium and kinetic calculations lies in the densities of S⁺, S₂⁺, SF⁺, F⁻ and S⁻ species between 2000 and 6000 K. The multi-temperature method gives results very different from those of the kinetic calculations even when neglecting the charge loss reactions. This is in contrast to what happens with H₂ or N₂.