Measurement of the ionisation and attachment coefficients in SF₆and air mixtures

The Townsend first ionisation coefficient alpha /p 20 , the electron attachment coefficient eta /p 20 , the effective ionisation coefficient alpha /p 20 (=( alpha /p 20 )-( eta /p 20 )) and the secondary coefficient gamma T for SF 6 and air mixtures have been measured by the steady-state Townsend method for 51<or approximately=E/p 20 <or approximately=160 V cm -1 Torr -1 (155<or approximately=E/N<or approximately=486 Td). The results show that the alpha /p 20 of the mixtures does not vary linearly with the fractional SF 6 partial pressure k and has a minimum value at a fixed value of E/p 20 . It is found that the eta /p 20 increase approximately linearly with k at each E/p 20 except at small k values for E/p 20 >or approximately=120 V cm -1 Torr -1 and the alpha /p 20 varies approximately linearly with k at E/p 20 =160 V cm -1 Torr -1 , but the linear relationship of alpha /p 20 with k tends to break down with decreasing E/p 20 . The results also show that the gamma T decreases rapidly from about 10 -3 to the values of gamma T for SF 6 with increasing k at fixed E/p 20 and the limiting E/p 20 , at which alpha /p 20 = eta /p 20 , rapidly increase with increasing k as those of the SF 6 and nitrogen mixture.