engleski

Coherent Selforganization of Target Surface in Laser-Matter Interactions

The evolution of shock-accelerated density-stratified interfaces is of considerable funadamenta and practical interest. However, the laboratory experiments that can generate such interfaces and vortex filaments as their instabilities, are difficult and usually not succeseful. We have demonstrated that laser-matter interactions (LMI) on ns-time scale can ( in specific case) generate such instabilities and vortex filaments as coherent self-organized structures. Usually, they appear as individual filaments, or helically paired, braided or tangled structures of various degree of topological comlpexity.(1-3) Here, we demonstrate that ns-LMIs with Indium target ( E ~ 80 – 100 mJ,  = 30 ns, 2R ~ 3 mm), generate the vortex filament bundles, as well as their merging into wavelet surface. Becouse of ultrafast cooling associated with termination of the laser pulse, these structures stay frozen permanently, thus enabling a posteriori analysis. The laser generated shock enters through the medium of density  1 striking the interface (contact surface) that represents the region of liquid Indium  2 , with  =  2 /  1 (high temperature and low temperature regions of the laser spot). The Richtmyer-Meskhow (RM) instability appears as the extension of the Rayleigh-Taylor (RT) instability, thus generating coherent vortex structures (vortex filaments). Formed on the interface, vortex filaments are stable for the « fast-slow» interface, if  > 1 (4). The scenario comprises 3 steps: (i) vorticity deposition in the inclined interface ; (ii) vorticity evolution , and (iii) vorticity merging. (i) The instantaneous angular displacement of the interface and the circulation along it, is deposited by the shock. After the shock has passed the interface, the essential feature is an angularly displaced translating contact surface, which bears a thin layer of vorticity. (ii) The vorticity evolution phase follows in which the vortex layer diffuses laterally as it rotates globaly. The ends of vortex layer begin to roll up. The roll up of the lower interface region proceeds as the vorticity « binds» with its mirror image (wall vortex). The negative vorticity causes the interface to rotate in opposite direction, ie. away from the wall. (iii) Merger of vortices, and another one ( cascade of mergers), occurs, resulting in the formation of surface undulations along the stretching interface layer. .

laser matter interaction; vortex filaments; coherent surface structures

nije evidentirano