Ionisation and attachment in binary mixtures of SF6-N2 and CCl2F2-N2

Abstract

Experimental results are presented of ionisation ( alpha ) and electron attachment (( eta ) coefficients evaluated from the steady-state Townsend current growth curves for SF6N2 and CCl2F2-N2 mixtures over the range 60<or=E/P<or=240 (where E is the electric field in V cm-1 and P is the pressure in Torr reduced to 20 degrees C). In both the mixtures the attachment coefficients ( eta mix) evaluated were found to follow the relationship; eta mix= eta (1-exp(- beta F/(100-F))) where eta is the attachment coefficient of pure electronegative gas, F is the fraction of the electronegative gas in the mixture and beta is a constant. The ionisation coefficients ( alpha mix) generally obeyed the relationship alpha mix= alpha N2(1-F/100)+ alpha A(F/100) where alpha N2 and alpha A are the ionisation coefficients of nitrogen and the attaching gas respectively. However, in case of CCl2F2-N2 mixtures, there were maxima in the alpha mix values for CCl2F2 concentrations varying between 10% and 30% at all values of E/P investigated. Effective ionisation coefficients ( alpha - eta )/P obtained in these binary mixtures show that the critical E/P (corresponding to ( alpha - eta )/P=0) increases with increase in the concentration of the electronegative gas up to 40%. Further increase in the electronegative gas content does not seem to alter the critical E/P.