From the first pulse to subsequent pulses: Evolution of discharge inside a preformed air bubble in water

The evolution of pulsed discharge inside a preformed air bubble in water from the first to subsequent pulses was experimentally investigated using a synchronized needle-bubble system. A positive nanosecond high-voltage pulsed power supply, together with a pulse valve and ICCD imaging, was used to generate reproducible bubbles and record discharge development with good temporal synchronization. Although the preformed bubbles showed good repeatability in size and morphology, the first-pulse discharge remained highly stochastic. The first discharge was mainly corona-like and was not significantly affected by bubble size once the electrode was fully covered by the bubble. With increasing pulse width and pulse number, the discharge evolved from corona-like emission to streamer discharge, accompanied by increasing bubble-interface instability, including wrinkling and eventual rupture. Increasing the solution conductivity significantly enhanced the discharge intensity, enlarged the luminous region, and promoted streamer propagation along the inner bubble surface. At sufficiently high conductivity, the first pulse already produced strong discharge and rapid bubble rupture. In addition, both current amplitude and energy dissipated per pulse increased with conductivity and pulse number. These results demonstrate that the discharge evolution inside a preformed bubble is jointly governed by pulse history, pulse width, and solution conductivity, and that residual effects from previous pulses play an important role in the transition from the first pulse to subsequent discharges.