Investigation of positive and negative modes of nanosecond pulsed discharge in water and electrostriction model of initiation

TL;DR

This study examines nanosecond pulsed discharges in water with both polarities, using optical diagnostics to analyze initiation and development phases, and proposes an electrostriction model for discharge initiation.

Contribution

It provides detailed optical diagnostics of nanosecond water discharges and introduces an electrostriction-based model for understanding initiation mechanisms.

Findings

Discharges develop similarly to those in long gaseous gaps.

Refractive index and density changes observed near electrode tip.

Electrostriction model offers explanation for initiation phenomena.

Abstract

This work investigates the development of nanosecond pulsed discharges in water ignited with the application of both positive and negative polarity pulses to submerged pin to plane electrodes. Optical diagnostics are used to study two main aspects of these discharges: the initiation phase, and the development phase. Nanosecond pulses up to 24 kV with 4 ns rise time, 10 ns duration and 5 ns fall time are used to ignite discharges in a 1.5 mm gap between a copper plate and a tungsten needle with radius of curvature of 25 um. Fast ICCD imaging is used to trace the discharge development over varying applied pulse amplitudes for both positively and negatively applied pulses to the pin electrode. The discharge is found to progress similar to that of discharges in long gaps in gases, both in structure and development. The more important initiation phase is investigated via Schlieren transmission imaging. The region near the tip of the electrode is investigated for slightly under-breakdown conditions, and changes in the liquids refractive index and density are observed over the duration of the applied pulse. An attempt to explain the results is made based on the electrostriction model of discharge initiation.