Measurement of Ionization and Attachment Coefficients in Sulphur Hexafluoride in Uniform Fields

The growth of pre-breakdown currents in uniform field conditions in sulphur hexafluoride at different pressures for values of E/p in the range of 90 to 160 v cm -1 mmHg -1 (5-200 mmHg pressure) indicates very large electron attachment. It is assumed that the mechanism of negative ion formation is due to dissociative attachment; therefore values of α and the dimensionally equivalent attachment coefficient η have been computed from the semi-logarithmic plots of current against electrode separation by employing the modified Townsend equation for the growth of current. Further, static breakdown potentials have been measured up to a value of pd (pressure × gap length) similar 400 mmHg cm, and a comparison with the data for dry air gives the relative dielectric strength of SF 6 as about 2.8 at 380 mmHg cm.