Time-resolved analysis of negative pulsed discharges in water: Streamer initiation and propagation modes

Analyses of the initiation and propagation-mode transitions of negative streamers are essential to develop a thorough understanding of underwater pulsed discharges. This paper presents experimental observations of negative streamers in deionized water performed using time-resolved visualization and electrical measurements. Based on the distributions of discharge inception delay and quantitative calculations of the ionization initiation fields, a streamer initiation model combining Zener ionization and field emission processes is proposed to account for discharges with short inception delays (<200 ns). Joule heating is the main mechanism responsible for discharges with an inception delay on the order of microseconds. The evolutions of bushy streamer morphology show that streamers exhibit more uniform structures under higher voltages, as most of the streamer branches grow simultaneously. A large-area discharge of bushy streamers is observed and is speculated to be closely related to the charge dynamics at the gas/liquid interface. The propagation of filamentary streamers and the transitions from filamentary to bushy streamers are presented. The redistribution of deposited charge and mitigation of the electric field during branching are the main causes for the mode transitions of streamers.