The Effect of Field Nonuniformity and Asymmetry on Ionization Current Growth Measurements in Sulfur Hexafluoride

Determination of ionization coefficients in SF6 and other electronegative gases from ionization current growth measurements is discussed, and an expression is derived for the influence of field nonuniformity upon the ionization current. The field distribution for Bruce-profiled electrodes in a typical experimental setup is calculated on a digital computer. The effect of the calculated deviation from uniformity of the axial field between the electrodes upon current growth in SF6 is determined. It is concluded that the length of the test gap should always be less than half of the normally recommended maximum gap length for Bruce-electrodes to avoid errors caused by field nonuniformity and asymmetry.