On the role of the electron impact ionisation and electron scattering cross-section in the breakdown strength of dielectric gases

Abstract

Discusses the role of the electron impact ionisation cross-section sigma i( epsilon ) and the electron scattering cross-section sigma sc( epsilon ) as a function of electron energy epsilon in the breakdown strength of gases/mixtures. Four gases (Ne, Ar, N2 and SF6) were chosen for which sigma i( epsilon ) and sigma sc( epsilon ) at low energies are known. Direct-current breakdown strength measurements on Ne, Ar, (Ne+SF6), (Ar+SF6), (N2+Ne+SF6) and (N2+Ar+SF6) were made and are reported, using sphere-on-sphere to sphere and square-rod to plane electrode geometries. On the basis of these measurements and the data on sigma sc( epsilon ) and sigma i( epsilon ) it was concluded that the magnitude of sigma sc( epsilon ) at subionisation ( epsilon <I) and especially at subexcitation ( epsilon <energy of lowest excited electronic state) energies is much more significant in effecting a high dielectric strength than is sigma i( epsilon ). The effect of perfluorination and static polarisability on sigma i( epsilon ) is also discussed.