Observation and modelling of vapor bubble and streamer initiation in water under long duration impulses

The inception of vapor bubbles and discharges in water under high voltage impulses of long duration (500 μs) is investigated experimentally, and by numerical simulation. Time delay to bubble and discharge inception are measured versus conductivity and voltage, using fast visualization and optical detection with a laser light diffusion system. A decrease of gap resistance versus time is first observed, and attributed to the heating of water due to Joule dissipation. With fixed conductivity and variable voltage, bubble inception is correlated to a nearly constant value of resistance, suggesting that a constant value of temperature is present when bubbles appear. This hypothesis is checked by numerical simulation.