A New Method to Generate Strong Underwater Shock Waves Using Water Electrolysis in Negative Streamer Pulsed Spark Discharge

In this paper, we propose a new technique to improve an energy efficiency in development of strong shock wave by using underwater pulsed spark discharges. By preconditioning a water-gap via water electrolysis, hydrogen microbubbles are produced at a cathode, and drift towards the anode during the preconditioning phase. A negatively enhanced electrode pair is adopted to initiate the negative (anode-directed) streamer in the presence of microbubbles near the cathode tip. When a high-voltage pulse is applied to the water-gap just after the end of the preconditioning phase, the overall pre-breakdown process is observed to be remarkably accelerated. Since the reaction rate of the electrolysis depends on the experimental conditions such as water conductivity, hydrostatic pressure, structure of water-gap, and amplitude of preconditioning pulse, it is found that there is an optimum preconditioning duration in terms of energy efficiency for the shock wave generation. More interestingly, it is experimentally demonstrated that the morphology of the underwater streamer significantly influences on the development of shock waves. In particular, the tree-like structure of the negative subsonic streamers is much more advantageous than the volcano-like structure of the conventional positive (cathode-directed) subsonic streamers. That is, it is clearly shown that this technique overcomes the biggest drawback of negative subsonic discharges, i.e. the slow vapor bubble formation due to screening effect, and thus provides a new way to improve the efficiency of shock wave generation by pulsed spark discharges in water. Here, we elucidate the working principle of this technique, and discuss some engineering issues associated with the environmental factors.