Effective ionization coefficients and electron drift velocities in gas mixtures of SF6with He, Xe, CO2and N2from Boltzmann analysis

The effective reduced electron ionization coefficients (α−η)/ N , with α and η denoting the first Townsend ionization and attachment coefficients, respectively, and the drift velocities are calculated for different mixture compositions of SF 6 with He, Xe, CO 2 and N 2 , by solving the electron Boltzmann equation, under the two-term approximation, in a steady-state Townsend discharge. It is shown that a self-contained equation for the isotropic component of the electron velocity distribution can be derived, in which the effects of the non-null divergence of the electron particle flow along the discharge and of the non-conservative collisions for the electron number density, i.e. electron attachment and ionization, are properly taken into account. The critical values of E / N for which the ionization exactly balances the electron attachment are determined. The effects produced by changing the electron energy partition Δ between the primary and the secondary electrons after an ionizing collision are evaluated. The calculated results obtained here are then compared with measured data reported in the literature for pulsed Townsend experiments.