Plasma-acoustic sources and their practical applications

Spark discharges in dielectric fluids result in the formation of highly conductive plasma-vapour cavities between the electrodes. These cavities have extremely high internal pressure (up to several GPa) and expansion velocities. In water for example, the spark plasma cavity can expand with a velocity of 100 to 1000 m/s. Such hydro-dynamic parameters make plasma transient cavities a useful source of high power ultrasound (HPU) pulses in dielectric liquids. The HPU pulses generated by the plasma discharges are extremely wide band and can be used in practical applications. The spark discharges can be generated by two approaches: using free discharges in the fluid and by placing a thin guide wire between the electrodes. Both free and wire-guided spark discharges have their advantages which are discussed in the paper. The paper also presents a phenomenological approach to the calculation of the dynamic resistance of the plasma in the cavity and pressure of the acoustic pulses radiated by the cavity. This model is based on the plasma channel energy balance equation used by Braginskii and links the hydrodynamic characteristics of the plasma with the parameters of the electric driving circuit. Several practical applications of the HPU pulses are also discussed.