Pulsed Nanosecond Discharge Development in Liquids and Nano-voids Formation

The dynamics of pulsed nanosecond discharge development in liquid water, ethanol and hexane were investigated experimentally. High-voltage pulses with durations of 20 and 60 ns and amplitudes of 6-60 kV were used for discharge initiation. It is shown that the dynamics of discharge formation fundamentally differ between liquids with low and high dielectric permittivity coefficients. In water (high permittivity), two phases were observed in the process of the discharge development. The first phase is connected with electrostriction compression of the media near the needle tip and the formation of a rarefaction wave in the surrounding liquid. The second phase (the discharge phase) has a pronounced start delay, which depends on the voltage of the high-voltage electrode. Thus, at low voltages, the pulse length is insufficient for the initiation of discharge, and the process consists of the first phase only, i.e., the formation of an electrostriction rarefaction wave. At higher voltages, the discharge start delay time decreases rapidly, and discharge commences simultaneously with the formation of hydrodynamic perturbations by the electrostriction forces present in the media. Shadowgraphic laser visualization of the process demonstrates the transition from a pure hydrodynamic density perturbation in the rarefaction wave to a developed streamer-leader process with a strong energy release in the channels and the formation of strong shock waves around the channels. Unlike in water, the first phase is essentially non-existent in liquids with low dielectric permittivity coefficients because of the small electrostriction forces and the low intensity of the rarefaction wave that is formed. The second phase in the process (discharge) begins at significantly higher voltages on the high-voltage electrode, immediately leading to the long branched structure of the streamer-leader flash. The difference in the nanosecond discharge development in liquid dielectrics may be explained by the formation of micro-discontinuities in the media during the electrostriction compression/rarefaction stage in liquids with high dielectric permittivities.