Measurement of the ionisation and attachment coefficients in SF₆and helium mixtures

The Townsend first ionisation coefficient alpha /p 0 , electron attachment coefficient eta /p 0 , effective ionisation coefficient alpha /p 0 (= alpha /p 0 - eta /p 0 ) and total secondary coefficient gamma T for SF 6 and helium mixtures have been measured by the steady-state Townsend method for 54<or=E/p 0 <or=215 V cm -1 Torr -1 (153<or=E/N<or=608 Td), together with the limiting E/p 0 , (E/p 0 ) lim , which represents E/p 0 for alpha /p 0 = eta /p 0 . The results show that the curves of alpha /p 0 and alpha /p 0 against the fractional SF6 partial pressure k at a fixed E/p 0 have a maximum for E/p 0 >or=86 V cm -1 Torr -1 . However, the value of alpha /p 0 and alpha /p 0 varies approximately linearly with k for E/p 0 <or=54. Tests at E/p 0 =107 and 161 V cm -1 Torr -1 for k near the peak did not show pressure dependence in the alpha /p 0 value. The eta /p 0 decreases with increasing E/p 0 at fixed k, and increases with increasing k at fixed E/p 0 , for k<80% for which eta /p 0 is deduced in the present work. It is also found that the gamma T for the mixtures decreases rapidly with k at fixed E/p 0 . The present value of (E/p 0 ) lim for SF 6 and helium mixtures increases approximately linearly with k, in contrast to SF 6 and N 2 mixtures with which it increases rapidly at small values of k. Possible explanations for the variation of alpha /p 0 and (E/p 0 ) lim with k, as mentioned above, are given.