Time resolved imaging of a pulsed plasma discharge in water

Plasma discharges inside water have been widely used for pulsed power applications such as water high voltage lines and switches. More recently underwater plasma discharges have been proved to be very efficient as an advanced oxidations process. The plasma inside water is a source of useful chemical compound such as hydrogen peroxide and hydroxyl radical that can be used for pollution control and sterilization of water. A global understanding of the physical mechanisms responsible for initiation and propagation of the several plasma discharges modes inside liquids is still needed. Several attempts have been made by several authors to explain the local energy deposition and the subsequent propagation of prebreakdown plasma filaments inside the liquid. The main physical problem is the density of the medium and its ability to endure electron avalanches. The electronic states description of liquids under very high applied electric field is not well understood. The electronic mobility is very low in a liquid compared to gas at atmospheric pressure due to higher elastic scattering and excitation rates and very high recombination probability. The main candidate mechanisms are thus charge injection in the liquid phase or bubble mechanism with a phase change prior to any electron avalanche. Here is reported a time resolved imaging study of several modes of plasma discharge in water. The initiation criterion and the propagation velocity of the discharge have been measured with fast emission imaging and shadow imaging. A time resolved spectroscopic study was performed to access the plasma parameters through line broadening of the hydrogen line and the OH line. A parametric study has been made and particular interest was given to the influence of the ionic conductivity of water. Compared to other insulating liquid, the case of water is quite special due to its low molecular weight, its high dielectric constant due to high polarisability, and it high conductivity. The ionic cond- uctivity is low for most of the studies performed in the pulse power community, whereas the advanced oxidation process using plasma will proceed at medium or saline conductivity. Here we report the influence of the ionic conductivity of the liquid on the morphology, the initiation, the propagation, and the plasma parameters such as the electronic density.