| Summary: | 碩士 === 國立中正大學 === 物理所 === 97 === The idea of laser-induced spherical particle explosion is designed for understanding the spatially unbound point explosion and for comparing with the shock wave model of a strong point explosion. An isolated micron-meter sized spherical carbon particle is ablated by a 10 nsec laser pulse in N2 background. The expansion dynamics has been investigated by schlieren technique. We found that the expanding diameter of shock wave are point explosion in ˆy (parallel to the laser direction) and cylindrical explosion in ˆx (perpendicular to the laser direction) which are fit with the shock wave model before 10 ms. Unfortunately, the idea of point explosion is failed by the initial geometric condition of focusing lens and the gas breakdown coupling. There is a peculiar property of the particle breakdown induced plasma, which looks like a peanut-shaped and it is different from the similar case of the shape of laser-induced pure gas breakdown, which looks like a stretched egg. The plasma core cools down to form hot gas in the time-lapse sequence. The hot gas expands, but collapses by an inward pending flow later. Two counter propagating flows enter the body of the hot gas along laser direction and collide in the body of hot gas which is observed with vertical knife-edge (along laser direction) schlieren system. The upstream flow (opposite to the incident laser direction) overcomes the downstream flow and then stretches out a ”tail” pointing to the focusing lens after 70 ms, where it forms a vortex structure. The collapsing and penetrating effects are induced by the interaction of the rarefaction wave and the peanut shape plasma core, which is indentified that this vortex structure is not caused by pressure, temperature gradient or thermal convection with ambient cool gas in the previous explanation. At 300 ms, the wavy pattern appears on the vortex structure, which should be induced by the Widnall instability.
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