Discharge processes in a laser ablation-induced cavitation bubble

The cavitation bubble is not a static bubble but has the dynamics of expansion, shrinkage, and collapse. In this work, we tried to produce electrical discharge in a cavitation bubble which was induced by laser ablation of a titanium target in water. We placed a needle electrode at a distance of 0.5 mm from the gas-liquid boundary of the cavitation bubble at the maximum size. The electrode was connected to a high-voltage pulsed power supply. The temporal variation of the bubble size was measured by shadowgraph imaging using a high-speed camera. The high-speed camera also captured the optical emission image from the discharge. At 50 $\mu$s after applying a pulsed high voltage, we observed the formation of a swelling from the cavitation bubble. The swelling was lengthened toward the needle electrode. We observed the electrical discharge between the needle electrode and the target when they are connected by the cavitation bubble and its swelling. It is noted that both the directions of the swelling lengthening and the discharge channel are perpendicular to the gas-liquid boundary of the cavitation bubble, suggesting that the electric field is perpendicular to the gas-liquid boundary because of the existence of movable electrical charges.