Analysis of the Current Waveforms Observed in Underwater Spark Discharges

Summary form only given. There is a growing interest in the industrial applications of high power ultrasound (HPU) generated using a spark discharge in a liquid medium driven by a pulsed power supply. These applications include: metal processing, in particular peening techniques to increase hardness and resistance to cracks, comminution of waste materials such as stainless steel slag, bottle glass and spent nuclear fuel assemblies, and the treatment of mineral ores. The output power of the HPU source depends critically on the electrical characteristics of the pulsed power supply and the dynamic characteristics of the plasma channel that the supply is driving. In general the overall circuit behaves as an under damped RCL circuit. By examining the characteristics of the current transient associated with the discharge, it is possible to derive data on the dynamic behaviour of both the resistance and inductance of the circuit. If data is available on the current transient associated with the operation of the supply driving a short circuit load, it is possible to separate the dynamic behaviour of the resistance and inductance of the power supply from those associated with the plasma channels. Once this has been performed it is possible to calculate the power delivered to the channel and its variation with time. This data can be used to optimise the operation of the HPU source and to test the validity of theoretical models of plasma channel development in liquids. This analysis has been performed on current transient data obtained for a range of spark gap lengths, and voltages. The dynamic behaviour of the channel resistance and inductance is discussed and possible explanations for the observed behaviour are considered.