Breakdown Process in Rod-to-Plane Gaps with DC Voltages

The breakdown process in rod-to-plane gaps, of which gap spacings were up to 80 cm under dc voltage, was investigated with multiple techniques using an image converter camera, five photomultipliers, and two still cameras, as well as with the measurements of voltage and current. The combination of a photomultiplier with an image converter camera allowed us to view for the first time the leader development with dc voltage. It is shown that breakdown proceeds, after well-known corona discharges, through streamers bridging the gap, leader development, and their bridging the gap to induce the intense return ionizing waves, to lead the formation of a spark channel. The charge density was estimated to be 2.7-24.5 pC/cm in corona discharges for the applied voltage of 170-410 kV and to be on the order of 10-8 C/ cm in the leader head.