High power ultrasound generation by supersonic electrical discharge in water

This thesis presents the experimental studies performed to optimise underwater supersonic plasma discharges for use as a source of high power ultrasound.

The method for generation of high power ultrasound using supersonic discharges underwater has been optimised according to four key system requirements: peak pressure, shot-to-shot repeatability, efficiency of electrical to acoustic energy conversion, and capability for high repetition rate operation. The research was performed with two different 100 kV pulsed power generators. The first pulsed power generator injected a total of 5.5 J of electrical energy across discharge electrodes in water (termed the ‘load’) with a minimum rise time of the voltage (10-90% of the peak value) of 250 ns. The second pulsed power generator injected total of 125 J of electrical energy with a minimum voltage rise time onto the load of 10-15 ns. 

The first generator was capable of producing powerful acoustic pressure from supersonic discharges in both deionised and tap water using very little electrical energy, and with comparable efficiency. For the same 5.5 J electrical supply, the peak acoustic pressure generated by operation of the system in tap water had a peak value 89% of the peak pressure generated during operation of the system in deionised water. The corresponding efficiency of energy conversion from electrical to acoustic for operation of the system in deionised water and tap water are 0.8% and 0.7% respectively.

An insulated pin-hemisphere electrode geometry was developed for generating reliable discharges and reducing the degradation of the electrodes, producing no significant change in performance over several hundreds of shots. Using this geometry, 4 mm was found to be the largest inter-electrode gap that produced a reliable electrical breakdown. And for inter-electrode gaps up to this distance, a larger inter-electrode gap produced a larger peak acoustic pressure. 

Two concepts for electrode geometry were developed to increase emitted pressure by increasing the volume of plasma in the gap. The novel geometries achieved this by facilitating the breakdown of a larger inter-electrode gap and by encouraging the simultaneous formation of multiple plasma channels. The following conclusions are tentatively presented. More simultaneous plasma channels result in a higher acoustic pressure. It is possible to increase the maximum breakable inter-electrode gap (and thereby increase maximum acoustic pressure) by providing an appropriate surface (with high dielectric constant) for the streamers to propagate along.

There was no appreciable increase in peak acoustic pressure emitted when operating the 125 J energy generator 2 when compared with the 5.5 J generator 2. Voltage signals applied to the load with a faster rise time generated larger acoustic pressures for the same size inter-electrode gap, and also facilitated the break-down of larger inter-electrode gaps. This second effect was shown to produce a more significant increase in peak pressure.

The 125 J generator was operated in a unique mode wherein the voltage applied across the load oscillated with high frequency and was strictly positive. With the fastest rise time (10 ns), corresponding with the highest frequency of oscillation (10 MHz), it was possible to generate a peak pressure 7.8 times larger than the largest pressure produced by the generator 1 system.

A novel variation of a Heterodyne Velocimetry system developed at Imperial College London was implemented to directly measure the local pressure conditions in the discharge region. Preliminary results suggest that a generator with higher energy and faster rise time produces a shock region with larger dimensions but a very similar pressure (~45 MPa).

Extrapolation of the findings in this last chapter suggest that with the implementation of a voltage signal with a shorter rise time, higher frequency of oscillation, larger energy supply and increased inter-electrode gap, it would possible to produce even larger acoustic pressures than have been reported here.