Transport coefficients in arc plasma of SF6–N2 mixtures

Abstract

We compute thermal and electrical conductivity coefficients as a function of temperature for different compositions of the N2–SF6 mixture. The method we used was derived from Chapman–Enskog’s. The composition of the plasma is calculated assuming the hypothesis of local thermodynamic equilibrium. Calculations were made for temperatures between 1000 and 15 000 K. We show that the densities of the compounds specifically due to the SF6–N2 mixture (i.e., NF, NS, N2F4, NF3, etc....) are negligible compared with the densities of the products due to SF6 alone and N2 alone. The characteristics of the transport coefficients of the mixture are therefore a combination (depending on the proportions in the mixture) of the characteristics of pure N2 and SF6. These theoretical values allow us to interpret the curves obtained experimentally by measuring, using laser interferometry at two wavelengths, the variation of electron number density in a decaying arc discharge in a SF6–N2 mixture. By substituting the values of the transport coefficients into an energy balance equation, we also determined, in the stationary state the profiles of the temperature and the electric field and the variation of the electron density with the discharge current.