The application of optical diagnostics to high energy electromagnetic acoustic transducers

This thesis is concerned with the design and construction of an electromagnetic acoustic transducer (EMAT) and the characterisation of its acoustic field both conventionally, using a hydrophone and with high resolution laser illuminated schlieren techniques. During the early 80s the introduction of the EMA T along with the other types of shock wave source used for lithotripsy, revolutionised the treatment of stone disease. The process of shock wave induced destruction of calculi and the use of shock waves in other areas of medicine will be discussed, along with the causes and effects of stone disease in man. For the first time high temporal and spatial resolution schlierenimages of the shock waves and there interaction with simulation kidney stones have been recorded. The technique provides a clearer picture of the fragmentation process and may assist research into the suitability of shock wave treatment in other areas of medicine currently under investigation. Schlieren studies of the acoustic field have shown the complex structure of not only the EMA T shock wave, but also that associated with cavitation in the field. The primary source of cavitation is due to the rupture and subsequent collapse of bubbles generated in the water by the strong rarefaction phase of the shock wave. The images give evidence for the interaction of these 'primary' cavitation shocks with bubbles in the field, the collapse of some of these bubbles giving rise to additional or 'secondary' cavitation shocks. An optical lensing effect introduced by the shock has also been investigated. Objects seen through or immersed in the field of an EMAT shock wave such as cavitation, appear highly distorted, due to the strong positive and negative lensing effects associated with the changing refractive index of the compression and rarefaction cycles of the shock wave.