Measurement of the effective ionisation coefficient in SF₆and air mixtures at high E/p₂₀values

The effective ionisation coefficient alpha /p 20 (= alpha /p 20 - eta /p 20 , alpha and eta being respectively the Townsend first ionisation coefficient and the electron attachment coefficient) and the total secondary ionisation coefficient gamma T for SF 6 -air mixtures have been measured by the steady-state Townsend method for 150 V cm -1 Torr -1 <or=E/p 20 <or=2000 V cm -1 Torr -1 . The results show that alpha /p 20 decreases with decreasing SF 6 percentage partial pressure k for E/p 20 <or=220 V cm -1 Torr -1 and increases with increasing k above this E/p 20 value. The alpha /p 20 -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/p 20 .