Abstract
The paper studies the positive surface discharges initiated from the triple junction at which a solid insulator, SF6 gas and a metal electrode meet together. The transitions of the surface discharges at impulse voltage applications have been observed by the optical measurements with a photomultiplier and an image converter camera. The measurements show that the streamer to leader transition time is in inverse proportion to the gradient of impulse front dV/dt and independent of the gas pressure P. We propose the physical model based on the precursor mechanism to explain the new scaling law. In the model the following phenomena are considered; (1) dependence of the ion current in a streamer channel on the gradient dV/dt, (2) photoelectric emission from insulator surface, and (3) pressure dependence of the effective gap length at the triple junction. The proposed model is also applicable to the scaling laws on the leader propagation. The leader step time is proportional to P-1(dV/dt)-1, and the propagation speed of leader depends on the products of the leader inception voltage and the gradient dV/dt.
