Negative ion motion in mixtures of CH4 and Ar with low SF6 content

This paper deals with the measurement of the mobility of negative ions in the mixture SF₆-CH₄-Ar, with SF₆ contents of up to 50%. The Pulsed Townsend Technique was used to produce electronic and ionic avalanches over the low E/N range in which ionization is either negligible or absent, and attachment processes are significant. The E/N range of measurement was 1 to 100 Td. The structure of the measured signals strongly suggests that there is only one negative ion, most likely formed by resonant attachment. A test of the measured mobilities with Blanc´s law provides further support to ascribe the drifting species to SF₆^-.