Bubble Growth Following a Localized Electrical Discharge and its Relationship to the Breakdown of Triggered Spark Gaps in Liquids

In order to investigate the mechanism of breakdown of the liquid-filled, triggered spark gap, a flash-illuminated, shadowgraph optical system has been used to photograph the pre-breakdown events in a triggered gap. Photographs indicate that in all cases the trigger spark is followed by the growth of a hemispherical bubble, or vapor cavity, and this bubble appears to be the precursor of the main gap breakdown. A theoretical investigation shows that the expansion and collapse of the cavity in the low-viscosity limit, follows a simple hydrodynamic model. We find that only about 1% of the circuit energy is transferred to kinetic energy of the liquid surrounding the expanding cavity. The time required for an expanding bubble to fill a 1 mm gap is of the same order as the breakdown time lag for the triggered gap. It is concluded that the bubble generated by the trigger spark clears the gap of liquid, leaving a low-density gas or vapor between the electrodes, so that the actual process of electrical breakdown takes place through the low strength gas, not through the liquid. In the case of longer gaps, of 2 mm and above, the bubble may have time to expand across the entire gap and it is suggested that an electrohydrodynamic instability may cause the bubble surface to breakup into streamers, which cross the gap and cause breakdown.