Electron-transport, ionization, attachment, and dissociation coefficients in SF6 and its mixtures

Abstract

An improved set of electron‐collision cross sections is derived for SF₆ and used to calculate transport, ionization, attachment, and dissociation coefficients for pure SF₆ and mixtures of SF₆ with N₂, O₂, and Ne. The SF₆ cross sections differ from previously published sets primarily at very low and high electron energies. At energies below 0.03 eV the attachment cross section is adjusted to fit recent electron swarm experiments, while the elastic momentum transfer cross section is increased to the theoretical limit. At high energies an allowance is made for the excitation of highly excited levels as observed in electron beam experiments. The cross‐section sets used for the admixed gases have previously been published. Electron kinetic energy distributions computed from numerical solutions of the electron‐transport (Boltzmann) equation using the two‐term, spherical harmonic expansion approximation were used to obtain electron‐transport and reaction coefficients as functions of E/N and the fractional concentration of SF₆. Here E is the electric field strength and N is the gas number density. Attachment rate data for low concentrations of SF₆ in N₂ are used to test the attachment cross sections. Particular attention is given to the calculation of transport and reaction coefficients at the critical E/N=(E/N)_c at which the ionization and attachment rates are equal.