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Investigation of stagnation layer dynamics of counterpropagating laser induced air plasmas: Numerical simulations vis-à-vis experimental observations

The interaction and evolution dynamics of two counterpropagating shockwaves generated by two collinear laser induced air plasmas of equal and unequal energies are presented. The formation of a stagnation layer during the interaction of the shockwaves from the two plasma sources, namely, S1 and S2 separated by a distance of 4 mm, were studied over a duration of 30 μs, using two-dimensional FLASH radiation hydrodynamic code. The stagnation layer formed is observed to be “hard” in the case of interaction of equal energy sources, S1 = S2. While in the case of unequal energy sources with S2 > S1, the stagnation layer is observed to allow the propagation of energy. For both the cases of equal and unequal energy sources, a maximum increase in temperature at the stagnation layer by ∼1.5 and 1.9 times, respectively, is observed due to the interaction of the expanding plasmas around the stagnation layer. The density and pressure in the unequal interaction case at the stagnation layer is observed to be more than that of the ambient conditions by 4.9 and 19 times, respectively. A clear visualization of shock front of high energy plasma source traversing through that of a lower energy source leads to a channel formation, followed by a jet-let along the laser axis due to density drag. The evolution of interacting plasma is compared with that of the evolution of individual sources. The simulations were validated by the experimental observations giving a one-on-one mapping of the spatiotemporal evolution of plasma and shock front.