Measurements of electric field distribution in surface discharge using Pockels based potential difference probe

A potential difference probe has been used for measurements of the electric field distribution of a surface discharge creeping along an insulating acrylic plate. The probe consists of an SLD, a Pockels crystal, optical fibers and a pair of needle electrodes. The probe has a capacitance of only 1±0.1 pF. The influence of this probe on the discharge during measurement has been examined by another Pockels sensing system. It was found to be negligible for data taken when the discharge has gone past the probe needle electrodes. The maximum electric field in the streamer part of the discharge seems to be independent of the thickness of the acrylic plate. The leader electric field, for a plate of 2 mm thickness, is constant for discharge development lengths past the probe needles of more than 15 mm, and increases linearly for lengths between 10 and 15 mm