Dynamics of laser induced micro-shock waves and hot core plasma in quiescent air

Abstract

We present our results on spatio-temporal evolution of laser plasma produced shockwaves (SWs) and hot core plasma (HCP) created by focused second harmonic (532Â nm, 7Â ns) of Nd-YAG laser in quiescent atmospheric air at f/#10 focusing geometry. Time resolved shadowgraphs imaged with the help of an ICCD camera with 1.5Â ns temporal resolution revealed the presence of two co-existing sources simultaneously generating SWs. Each of the two sources independently led to a spherical SW following Sedov-Taylor theory along the laser propagation direction with a maximum velocity of 7.4Â km/s and pressure of 57Â MPa. While the interaction of SWs from the two sources led to a planar SW in the direction normal to the laser propagation direction. The SW detaches from the HCP and starts expanding into the ambient air at around 3Â Âs indicating the onset of asymmetric expansion of the HCP along the z-axis. The asymmetric expansion is observed till 10Â Âs beyond which the SW leaves the field of view followed by a deformation of the irradiated region in the XY-plane due to the penetration of surrounding colder air in to the HCP. The deformation in the XY-plane lasts till 600Â Âs. The dynamics of rapidly expanding HCP is observed to be analogous to that of cavitation bubble dynamics in fluids. Copyright © Cambridge University Press 2013.