High-speed optical and statistical investigations of prebreakdown phenomena in liquid dielectrics in the nanosecond region

High-speed multiframe laser schlieren recording and statistical investigations at various external pressures were carried out to study the formation of electrical discharges in liquids with ~50-300 ns application of voltages. The existence of different mechanisms for breakdown from the anode has been revealed. In n-hexane under increased pressures the ionisation mechanism is predominant. The higher pulse electrical strength of transformer oil is due to relatively low velocities of the streamer processes. In the case of micro- and submicrosecond electrical discharges from the cathode the structure of prebreakdown phenomena, mechanisms and velocities of their evolution (⩽10⁵ cm/s) are approximately the same. The results support ionisation and bubble mechanisms of the breakdown from the anode and cathode, respectively