Measurement of ionisation and attachment coefficients in difluorodichloromethane and SF₆gas mixtures

Abstract

The Townsend first ionisation coefficient alpha and the electron attachment coefficient eta were measured in CCl2F2/SF6 gas mixtures using the steady-state Townsend method over the range 90<or=E/p<or=130 V cm-1 Torr-1. The effective ionisation coefficient alpha of CCl2F2/SF6 exhibited a minimum around the mixture ratio of 50/50 indicating strong eta synergism ( eta =- alpha ) in the gas mixture. The limiting E/p values derived from the pre-breakdown ionisation current measurements in CCL2F2/SF6 mixtures were in agreement with the results of static breakdown experiments reported by previous investigators. However, contrary to the Boltzmann equation analysis by Okabe and co-workers (1985), the present study shows that the synergism is caused mainly by the change in alpha /p resulting from gas mixing rather than the change in eta /p.