Time-resolved imaging of pulsed positive nanosecond discharge on water surface: plasma dots guided by water surface

Abstract

Pulsed nanosecond discharges at atmospheric pressure produce non-thermal plasmas that can be used in various applications. The dynamics of such discharges are highly dependent on experimental conditions, particularly the propagation medium. In this study, pulsed nanosecond discharges in air in-contact with deionized water are investigated, and the dynamics of plasma emission are studied using an ultrafast imaging technique. Depending on the magnitude of the applied voltage, two discharge modes are observed: (i) highly-organized filaments and (ii) intense and less-organized plasma filaments that superimpose to the organized ones. Based on the acquired 1 ns resolved images, the highly-organized filaments can be considered as plasma dots that propagate at the water surface with velocities in the order of hundreds of km s−1. Detailed analyses of the dots number, by imaging, and of the discharge properties, by current–voltage characteristics, reveal that the charge of each dot is constant (3–5 nC), irrespective of the experimental conditions. After being compared with the plasma bullets, usually produced by jets, the analyzed dots are proposed as plasma quanta.