Visualizing micro-air blasts using double exposure holographic interferometry

Micro air blasts are generated over planar and corrugated surfaces, at stand-off distances of 4 and 11 mm, by focusing a 1.38J, 18 ns Nd:Glass laser pulse in a tiny spot of 300 mum diameter, under ambient conditions. This triggers the breakdown of air at optical frequencies with subsequent formation of laser plasma, which generates an outward propagating spherical micro-blast wave. The velocity of this wave decreases as time elapses, since the energy deposition is finite, with reference to both space and time. The evolution, propagation and subsequent reflection of the spherical micro-blast wave, from planar and corrugated surfaces are experimentally investigated, using double exposure holographic interferometry. One planar surface and four surfaces with varying degree of surface corrugations are used in this study. Mach reflection of the blast wave is observed at a stand-off distance of 4 mm over a planar obstacle while only regular reflection is visualized at an 11 mm stand-off distance. However only regular reflection is observed at both stand-off distances, in the case of corrugated surfaces.