Insulating properties of mixtures of nitrogen with electronegative gases

Calculations of the transport coefficients of insulating electronegative gases and their mixtures were performed under the conditions of the Townsend steady-state experiment. The Boltzmann equation was used for evaluation of the unknown collision cross sections by fitting calculated ionization coefficients, drift velocities and diffusion coefficients to experimental data. A stainless-steel vacuum chamber (10⁻⁸ torr), provided with a pair of Rogowski-profile electrodes, was set up and used to carry out a series of measurements. The ionization and attachment coefficients were evaluated from the steady-state prebreakdown currents. Results for SF6 are presented in this paper. A comparison between calculated transport coefficients and available experimental data is presented for the range of reduced fields between 70 and 220 Vcm⁻¹ torr⁻¹, using SF6, c-C4F8, and CCl2F2 as the attaching components. Observed discrepancies, mainly at low partial densities of the electronegative gases, are discussed: the experimental attachment coefficients, in particular, are considerably higher than the theory predicts for N2-SF6 and N2-CCl2F2 mixtures. Possible enhancement of ionization and attachment coefficients resulting from heavy particle interactions, such as Penning ionization or collisions between nitrogen compound states and electronegative molecules are examined. Relatively good agreement is noted between calculated and measured values of the limit field.