DC corona in SE/sub 6/: evidence of an electronic component for the negative polarity

Current-voltage characteristics of corona discharge properties in SF/sub 6/ have been investigated using a point-to-plane geometry. The hemispherical tip of the rod is 1 mm in diameter and the gas pressures are typically low (from 13.3 to 40 kPa). The measured ratio of the negative to the positive ion mobility varies with the gap length, converging as the point-to-plane distance increases (up to 7 cm for the lowest pressure). At short gap (<10 mm) and for the negative polarity, the measured current exceeds the saturation-current limit (unipolar) over a substantial voltage range before breakdown; the variation of the square root of the corona current is found to be linear with the applied voltage. The above observations are supporting evidence for an electronic component at small distance for the negative-polarity case. The estimated electronic contribution to the total current decreases with increasing gap length and pressure.<>