Measurement of the effective ionisation coefficient in SF6 and air mixtures at high E/p20 values

Abstract

The effective ionisation coefficient alpha /p20(= alpha /p20- eta /p20, alpha and eta being respectively the Townsend first ionisation coefficient and the electron attachment coefficient) and the total secondary ionisation coefficient gamma T for SF6-air mixtures have been measured by the steady-state Townsend method for 150 V cm-1 Torr-1 <or=E/p20<or=2000 V cm-1 Torr-1. The results show that alpha /p20 decreases with decreasing SF6 percentage partial pressure k for E/p20<or=220 V cm-1 Torr-1 and increases with increasing k above this E/p20 value. The alpha /p20-values of the present mixtures are summarised in a form of formula for the prediction and simulation of the ionisation current development and breakdown in the mixtures. The inverse current plots according to Gosseries are studied for the present measurements, but they do not give any evidence of pronounced non-equilibrium which could cast doubt on the validity of the measurement at high E/p20.