Modelling of electron multiplication mechanism in nanovoids initiating the breakdown in liquid water

The explanation of discharge initiation in liquid water by a nanosecond electric pulse poses a challenge, both theoretically and experimentally. The short duration of the pulse does not allow for a conventional gas breakdown initiated by electron avalanche – the liquid water does not have enough time to heat to form water vapor. Therefore, alternative mechanisms that operate on sub-nanosecond and sub-micrometer scales are assumed to be present. One of the prevailing hypotheses explains the initiation of the breakdown by the generation and expansion of nano-cavities due to strong negative pressure caused by the inhomogeneous electric field around the electrode. In this work, we present a combination of modeling approaches directed at analyzing the processes that would govern the discharge initiation. In particular, we use fluid dynamics to model the propagation of negative pressure in the liquid. The resulting negative pressure values are then used for the simulation of the cavity growth. Finally, the electron multiplication occurring in the cavities is modeled using the particle-tracking GEANT4-DNA software. The combination of methods allows us to peek into the time-resolved dynamics of discharge initiation in cavitating water.  The results reveal secondary electron gain per primary electron per volume for various voltage pulses and electrode shapes and are compared with published experimental observations.