engleski

Laser-generated Richtmyer–Meshkov and Rayleigh–Taylor instabilities. III. Near-peripheral region of Gaussian spot

Dynamics and organization of laser generated three-dimensional (3D) Richtmyer-Meshkov (RM) and Rayleigh-Taylor (RT) instabilities on metal target in the semiconfined configuration (SCC) are different in the Central Region (CR) (Lugomer, 2016a), Near Central Region (NCR) ((Lugomer, 2017) and the Near Periphery Region (NPR) of the Gaussian-like spot. The RMI/RTI in the NPR evolve from the shock and series of reshocks associated with lateral expansion and increase of the vapor density, decrease of the Atwood number and momentum transfer. SEM micrographs show irregular (chaotic) web of the base-plane walls, and mushroom spikes on its nodal points with disturbed 2D lattice organization. Lattice disturbance is caused by the incoherent wavy motion of background fluid due to fast reshocks which after series of reflections change their strength and direction. Reconstruction of the disturbed lattice reveals rectangular lattice of mushroom spikes with p2mm symmetry. The splitting (bifurcation) of mushroom diameter distribution on the large and small mushroom spikes increases with radial distance from the center of Gaussian-like spot. Dynamics of their evolution is represented by the orbits or stable periods in 2D phase space. The constant mushroom diameter - stable circulation or the stable periodic orbits - are the limit cycles between the unstable spiral orbits. Those with increasing periods represent supercritical Hopf bifurcation, while those leading to decrease and disappearance represent subcritical Hopf bifurcation. The empirical models of RMI, although predict dependence of the growth rate on radial distance (distance the reshocks travel to reach the interface), show many limitations. More appropriate interpretation of the simultaneous growth and lattice organization of small and large spikes gives the fundamental model based on the interference of the perturbation modes depending on their amplitude, relative phase and the symmetry. The late-time instability in the base-plane evolves into line solitons, vortex filaments and wave-vortex structures with chaotic rather than stochastic features.

Laser ablation ; Richtnyer-Meshkov instability ; Rayleigh-Taylor instability, small and large mushroom spikes, Hopf bifurcation, Lattice of mushroom spikes ; solitary waves ; wave-vortex phenomena

Published by Cambridge University Press