Impulsive breakdown in water: Optimisation of energy delivery for high acoustic output
The high voltage impulsive breakdown process in water is complex, with the nature of the impulsive breakdown depending upon the magnitude, polarity and rise time of the HV impulses, the water conductivity, and the electrode topology. In the case of μs and sub-μs high voltage impulses of sufficient magnitude, the breakdown develops through the formation of plasma streamers in the water. When the first streamer crosses the entire inter-electrode gap, the energy released in the breakdown channel transforms this channel into a gas/vapor cavity, which pulsates and radiates acoustic impulse(s). Optimisation of the hydrodynamic (period of cavity oscillation) and acoustic (peak magnitude of the acoustic impulse(s)) parameters is required for practical applications of these underwater spark discharges. The present paper analyses the functional behavior of the period of cavity oscillation and the peak magnitude of the acoustic impulse for spark discharges generated by self-triggered underwater discharges (free discharges), spark discharges triggered by air bubbles injected into the inter-electrode gap, and wire-guided discharges. The advantages and limitations of these methods of generation of underwater acoustic impulses by spark discharges are discussed.