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A new optical sensing technique for high power ultrasound

Woolsey, G. and Turnbull, S.M. and MacGregor, S.J. (2001) A new optical sensing technique for high power ultrasound. In: 2001 IEE Symposium on Pulsed Power, 2001-05-01 - 2001-05-02.

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The extremely rapid formation and expansion of a plasma channel created between two electrodes in a fluid, such as water or oil, results in the emission of a powerful radial shock wave that propagates out into the surrounding fluid. This shockwave is essentially a wideband high power ultrasound (HPU) pulse that can be utilized to perform a number of different tasks including applications associated with marine bio-fouling removal and bio-fouling prevention [l], materials reduction for mineral extraction and recycling [2], pipe cleaning, and lithotripsy [3]. These potential applications have been recognized for many years and a number are well documented. However, it is only through recent advances in pulsed power technology, that HPU is being developed for commercial use. A number of authors have reported the successful generation of pressure fronts of between several hundred bar and several kilobar [2,4]. However, using existing hydrophone technology to provide consistent measurements of this intense shock front has proved extremely difficult. The difficulties arise for several reasons. Firstly, without any focusing by the electrode assembly, the shock front intensity falls off rapidly with distance from the plasma channel. In order to measure the peak pressures generated, the pressure sensor has to be able to operate very close to the plasma discharge. It therefore has to be extremely robust and capable of withstanding a fast rising (several ps) high pressure fronts. Secondly, the sensor is adjacent to the rapidly changing voltages and currents used to generate the expanding plasma (typical Wdt value of around 1012A s-'). It must therefore to be electrically screened to minimise capacitive coupling (which can destroy standard hydrophones) and noise pick-up. Thirdly, at very close distances to the plasma discharge, the frequency content of the shock front is very wide and therefore the pressure sensor must possess a wide bandwidth (typically lOkHz - 1MHz). A method of overcoming the problems associated with measuring a short-lived HPU pulse is to detect optically the change in refractive index, which is caused by the change in density of the medium during the passage of the shock front. Optical techniques such as the schlieren method have been successfully used in gases, where the medium compresses readily and substantial changes in number density occur. But in a less compressible liquid, these techniques are not as useful as the change in number density is small, even for the pressure rises of several kilobar occurring across a plasma-generated shock front. An altemative optical approach is adopted here.