Negative ion motion in the mixtures of SF6 with CF4 and CH4-Ar

This paper deals with the measurement of the mobility of negative ions in the mixtures of SF6 with CF4 and the CH4-Ar (50:50) binary mixture with SF6 contents up to 50%. The pulsed Townsend technique was used to observe the integrated ionic avalanches over a range of the density-reduced electric field E/N for which ionization is either negligible or absent, and attachment processes are significant, leading to the formation of mostly SF-6. The E/N range of measurement was from 1 to 70 Td (1 Td=10(-17) V cm(2)), over which the measured mobilities were found to be almost constant. The mobility of the negative ions was also measured for trace amounts of SF6 in CH4 and Ar and 1% CF4, thereby providing a good value of the mobility of SF-6 in these pure gases, in order to test the measured mobilities with Blanc's law. We have found good agreement, within quoted experimental uncertainties, between calculated and measured values.