An electron multiplication mechanism at the early stage of nanosecond pulsed breakdown in water

Nanosecond and subnanosecond pulsed breakdown in water as potential plasma sources have attracted research attention in the past decade. In previous studies, electrostrictive cavitation has been established as the mechanism for breakdown initiation at nanosecond timescale, and electron dissociation from hydroxide has been demonstrated to be the most probable source of primary electrons in bulk pure water. A missing link between the cavitation inception and the formation of the first plasma channel is how the primary electrons multiply in the system. To answer this question, a clearer picture of the cavitation zone that sets the stage for electron multiplication would be needed. In this work, we present a new modeling framework and some numerical results of cavitation dynamics within the first nanosecond. The typical radius and number density of the generated cavities are calculated. The results are in support of the following hypothesis of electron multiplication: an electron gains energy while collisionlessly traversing a cavity under electric field; it causes collisional ionization at the opposite side of the cavity; the two resulting hydrated electrons migrate toward the next cavity and are released into the cavity upon arrival, thus completing one multiplication step.