Measurement of electron transport and effective ionization in SF6–air and SF6–O2 mixtures

Abstract

The electron drift velocity ve, the density-normalized longitudinal diffusion coefficient NDL, the density-normalized effective ionization coefficient (α − η)/N and the limiting field strength E/Nlim have been measured for the binary mixtures SF6–air and SF6–O2 for SF6 concentrations ranging from 1% to 50%. The overall reduced electric field strength E/N was varied between 1.2 and 500 Td (1 Townsend = 10−17 V cm2). A pulsed Townsend technique was used to perform these measurements, which turned out to be sensitive enough to distinguish relatively small differences in the electron drift velocities and longitudinal diffusion coefficients over the mixture range 1 to 50% SF6 in SF6–air and SF6–O2. On the other hand, the effective ionization coefficients show a well-defined variation with the SF6 content in the mixture. The limiting value of E/N at which the effective ionization coefficient is zero, that is, the critical field strength, turned out to be smaller than that for the SF6–N2 mixture. However, the difference in behaviour between SF6–air and SF6–N2 is small. To the best of our knowledge, we are not aware of any previously published measurements of these parameters for these mixtures.