Dielectric properties for SF6 and SF6 mixtures predicted from basic data

Abstract

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric-field–to–gas-density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron-beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self-consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6-He and SF6-N2 mixtures. Published experimental values of α for the SF6-He mixtures lie between the values of α calculated with and without ionization of SF6 by excited He atoms. Published experimental values of (E/N) * agree with our calculations to within 5% in both the SF6-He and the SF6-N2 mixtures.

Properties of highly electronegative plasmas produced in a multipolar magnetic-confined device with a transversal magnetic filter

“…SF -, which are very large for T eff <1 eV [55]. For 150 mA and a SF 6 flow of 0.5 sccm, we could obtain a negative ion to electron density ratio of 300 in a continuous operation regime without filtering out the electrons.…”

Towards a fully kinetic 3D electromagnetic particle-in-cell model of streamer formation and dynamics in high-pressure electronegative gases

“…For example, the excitation cross section for SF 6 is not well known, 34 although the threshold is known to be around 10 eV (Ref. 35). But the transport properties, such as drift velocity, temperature, and effective ionization rates of electron swarms in a gas can be readily measured.…”

“…We include one lumped excitation channel with threshold energy of 9.8 eV (see discussions regarding estimated values in Refs. 35,36). For this process, the incident electron loses the threshold energy and is isotropically scattered.…”

“…So, both the functional shape and magnitude of the cross sections must be adjusted to match the transport coefficients over some range of applied electric field values. Numerical values of the transport coefficients can be calculated by a Boltzmann code 35,36,41 in which the general electron distribution function in velocity space is approximated by a series expansion in a finite number of spherical harmonics.…”

Electron Collisions in Gas Switches*

“…In Fig. 1 are shown our recent measurements on the electron attachment rate constant k for SF 6 in the buffer gases N 2 , Ar, and Xe over a wide range a of E/N values. In Fig.…”