Photomultiplier and Field Probe Studies of Positive Point Plane Discharge Development in SF6/N2 Mixtures

Leader instability processes appear to be crucial in deciding the critical pressure Pc at large gap d to electrode radius r ratios. In order to study these instabilities, the internal electric fields and effective tip radii of prebreakdown leader/stem discharges have been measured in a point plane gap. The measurements were carried out using positive 1/50 ¿s impulse voltages in SF6 and mixtures of SF6 with N2. A dual photomultiplier system and a cathode field probe were used to make the measurements. A simple electrostatic model was used to infer the leader/stem internal fields from the measured quantities. Reasonable agreement with early, more restricted, work was obtained but some evidence that leader properties vary with their method of formation is presented. The measured leader fields as a function of pressure are shown to suggest that electrostatic instabilities may lead to a limiting value for Pc at large values of d/r.