Underwater spark discharge

The application of microsecond and sub-microsecond electric impulses of sufficient magnitude to water triggers a wide range of pre-breakdown processes which may include the formation of ionisation waves and gas-filled cavities. These processes can lead to the development of an intensive plasma-filled channel, which expands due to Joule heating of the hot ionised gases inside the channel and drives a sound wave into the surrounding water. Plasma spark discharges can be used as acoustic underwater sources for industrial applications, including material and mineral processing, waste treatment and sonar systems. From a different perspective, acoustic emissions from spark discharges in water-filled plasma closing switches employed in pulsed power systems can be considered as an energy loss mechanism. Therefore, further investigation of the mechanisms of electrical breakdown in water will help to optimise existing technologies and may result in the development of new practical applications of plasmas in water.

In the present paper the details of pre-breakdown mechanisms in water are reviewed, and breakdown volt-time characteristics for liquid water are discussed. The paper reports on both calculations and experimental measurements of the transient characteristics of the spark channel. Using an analytical model which links the hydrodynamic parameters of this channel with the electrical parameters of the driving circuit, the characteristics of acoustic impulses emitted by the spark have been obtained and compared with experimental data.

Finally, several practical applications of underwater spark discharges are discussed: plasma disinfection of water, high power ultrasound treatment of metal surfaces, plasma comminution of minerals and solid waste products, and plasma channel drilling for oil remediation.