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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 grounded 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 and its subsequent breakdown. An alternative approach is to use fast Joule evaporation of a thin metallic wire placed between the electrodes. The latter method could potentially provide a higher efficiency of conversion of the electrical energy stored in the high-voltage, pulsed power supply into acoustic energy radiated into the liquid as compared with free discharge HPU sources. This would be an advantage in practical applications of spark discharge HPU pulses. The present paper discusses the generation of the acoustic pulses in water by wire-guided spark discharges and describes a phenomenological model which allows the parameters of acoustic emission from these discharges to be obtained. Wire-guided spark discharges with energies up to 1 kJ have been generated by a capacitor-based pulsed power circuitry in a plastic tank tilled with tap water. The acoustic pulses have been monitored by Pinducer sensors. Also, the voltage drop across the discharge, and the electrical current flowing through the discharge have been recorded. 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. A phenomenological model has also been developed for the cylindrical plasma channel which is created by fast Joule evaporation of the wire and the consequent gas breakdown of the inter-electrode gap spacing. This model enables the calculation of velocity of the plasma channel expansion and the pressure of the radiated acoustic pulse. The experimental and analytical 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 optimisation of the parameters of wire-guided spark discharges. This data and the phenomenological model will be of practical interest for the development of industrial applications of such discharges.