Shadowgraphic Analysis and Speckle Photographic Measurement of a Micro Shockwave Induced by Laser Irradiation on a Metal Surface

Laser induced streaming is one of the interesting phenomena. When we provide a highly concentrated energy in a small region by laser beam, an induced streaming, such as micro shock wave or laser induced thermal acoustics is observed in the test region. In the present paper, a micro shock wave in air which was induced by laser irradiation was observed by shadowgraphic technique, and the shock Mach number was measured from the flow images. The induced density behind the shock wave was measured by speckle photographic technique. While the induced shock wave is produced in general by the ablation of a metal or gas breakdown due to the high density energy of laser irradiation, the present paper focused on relatively low density energy domain. As the laser power decreasing, the plasma was not observed in the test field but the micro shock still induced. The Mach number of this shock wave was measured by shadowgraphic images. For the shock measurement, a speckle photographic observation was carried out to obtain the density distribution behind the shock wave. The shock Mach number and density distribution was compared with self-similar solution in the moderate range of laser irradiation.