Generation of acoustic waves by focused infrared neodymium-laser radiation
thesisposted on 09.10.2012, 13:59 by Barry Ward
When the radiat Ion from a sufficient Iy powerful pulsed laser Is focused into transparent gaseous, liquid or solid media, dielectric breakdown may occur around the beam waist giving rise to a shortI ived high-temperature plasma which quickly heats the surrounding material. As a consequence of various energy-coupling mechanisms, this phenomenon causes the emission of one or more high-frequency ultrasonic acoustic waves whose speeds of propagation are dependent upon the physical propert les of the host medium. In the high-speed photographic studies described In this doctoral thesis, the 1.06-l'm near-Infrared radlat Ion from an 8-ns, lO-mJ Q-swl tched Nd:YAG laser Is focused In or onto a variety of fluid and solid materials. The rapid variations In density around the resulting plasma events are visualised using a Mach-Zehnder Interferometer with a sub-nanosecond dye-laser light source and a vldeo-Imaglng system. Calculat Ions of the corresponding transient pressure distributions are then enacted from the digitally-recorded Interferograms using a semi-automat Ic procedure under the control of a personal computer. Measurements of position. displacement and velocity are also carried out using the same opt ical apparatus In schlleren and focused shadowgraph highspeed photographic arrangements. The experimental work outlined In the following chapters is divided Into three broad fields of Interest. In the first of these, a study of the laser-generation of spherical shock waves· in atmospheric air is carried out. In the second, the neOdymium-laser beam Is focused onto different solid-fluid Interfaces resulting In the formation of bulk longitudinal and shear waves and surface acoustic waves. The Interactions of these waves with various obstacles and defects are Investigated with reference to their application to non-destructive test Ing. [n the third and most Important field, a detailed study of the dynamics of laser-Induced cavitation bubbles In water Is carried out. With regard to the associated phenomenon of cavitation erosion, particular attention Is paid to the formation of spherical acoustic waves by radlally-oscillatlng vapour cavities at various distances from transparent rigid boundaries.