Imaging Electric Breakdown over the Rise and Fall of ns Pulses in Water and Free-flowing Bubbles

Gas bubbles in water have been a target of extensive study for plasma discharges in liquid environments. The initiation mechanism of electric discharge in liquids was contested between the ‘bubble model’ and the ‘direct impact ionization model’ early on, debating whether a vapor phase must form prior to discharge [1]. Recently, a theoretical model of electrostrictive cavitation leading to nanovoids has been developed to account for the initiation of electric breakdown in liquids with fast rise-times of the applied voltage in inhomogeneous electric fields [2]. We show experimentally that even in the presence of a macroscopic bubble, discharge is first observed at the sharp tip of a positive electrode, within the first 4-8 ns of a 20 kV nanosecond pulse. Imaging of the electric breakdown was carried out over the 30 ns “half-wave” of the pulse and beyond with 2-10 ns image gate-widths to maximize temporal resolution. The short rise-time of the applied voltage, and the time it takes for the discharge to bridge the gap between the tip of the sharp electrode and the apex of the bubble, suggest the cavitation mechanism for discharge initiation and propagation in the water.

[1] Klimkin, V. F., and S. M. Korobeynikov. Impulse breakdown of liquids. Springer Science & Business Media, 2007.

[2] Shneider, Mikhail N., and Mikhail Pekker. Liquid dielectrics in an inhomogeneous pulsed electric field. IOP Publishing, 2016.