Measurements of attachment coefficients and ionic mobilities in SF6-nitrogen mixtures over the low-energy range 1.2-4.0 eV

Abstract

Measurements have been made of attachment coefficients alpha a, and ionic mobilities in SF6-nitrogen mixtures over the E/N range of 6.06<or=E/N<or=151.5 Td using a pulsed drift tube. Very low concentrations of SF6 are used in the mixtures and it is assumed that the electron energy distribution in the mixtures is the same as that in nitrogen. Values of alpha a/(SF6) are reported over the mean electron energy range of 1.2 to 4.0 eV and are found to decrease rapidly from a value of 4.55*10-15 cm2 ( alpha a/pSF6=150 cm-1 Torr-1) at E/N of 6.06 Td (corresponding to mean electron energy of 1.2 eV) to a value 1.37*10-16 cm2 ( alpha a/pSF6=4.5 cm-1 Torr-1) at E/N of 151.5 Td (corresponding to mean electron energy of 4.0 eV). The mobility of negative ions in SF6-nitrogen mixtures is found to remain constant over the entire E/N range covered, and a zero-field reduced mobility of 1.90+or-0.05 cm2 V-1 s-1 is obtained. The two-body attachment rate is estimated and is found to increase initially from a value of 6.1*10-9 cm3 s-1 at E/N of 6.06 Td ( epsilon =1.2 eV) to a peak value of7.8*10-9 cm3 s-1 at E/N of 34.4 Td ( epsilon =1.9 eV) and decreases thereafter. It is concluded that mean electron energy in SF6-nitrogen mixtures is very similar to that in pure SF6 and values of attachment coefficients alpha a, for any SF6-nitrogen mixture can be readily predicted from the present data.