High power ultrasound impulses induced by wire-guided spark discharges in water

Electrical discharges in liquid radiate wide-band high power ultrasound (HPU) pulses which are the result of the initial expansion and later collapse of a transient plasma/vapor-filled cavity located between the high voltage and ground electrodes. The cavity may be formed by a process of a fast streamer breakdown, or by a slower Joule heating and evaporation mechanism which leads to the development of an initial inter-electrode gas bubble which subsequently breaks down. An alternative approach is to use fast Joule evaporation of a thin metallic wire placed between the electrodes. Compared with free discharge HPU sources this method could potentially provide a higher efficiency for the conversion of the electrical energy stored in the high-voltage, pulsed power supply into acoustic energy radiated into the liquid. This would be an advantage in practical applications of spark discharge HPU pulses. The present paper discusses the generation of acoustic pulses in water by wire-guided spark discharges. Wire-guided spark discharges with energies up to 1 kJ have been generated by capacitor-based pulsed power circuitry in a plastic tank filled with tap water. A ball deformation gauge has been used to estimate the peak pressure in the acoustic pulses by measuring the acoustically-induced mechanical deformation of a copper sphere. The experimental results of the present study provide a general picture of the functional behavior of the peak pressure in the HPU pulses and will allow the optimization of the parameters of wire-guided spark discharges. This data could be of practical interest for the development of industrial applications of such discharges.