The interaction of two counterpropagating plasmas and shockwaves due to a 7 ns laser induced breakdown (LIB) of ambient atmospheric air was studied using the 2D emission and shadowgraphic imaging technique. The effect of separation distance (d) between the two plasma sources and their energy ratio on plasma and shockwave evolution is presented. These low density and low temperature plasma sources form a stagnation layer in the interaction zone that showed an interesting dependence on the input energy of the sources. The interaction dynamics of the two counterpropagating plasma plumes and the shock waves in ambient air are compared with that of the respective single plasma source to explicitly bring out the energy exchange during the interaction. The evolution of a plasma jetlet, studied by 2D emission images and shadowgraphic imaging, has shown very good correlation giving valuable insight into the role of a plasma induced shockwave on the expanding plasma. The propagation of the shock front of the high energy source through the plasma of a low energy source is observed to predominantly lead to the formation of a jetlet and its evolution.

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.

We present results on the dynamics of laser-induced blow-off shockwave generation from the rear side of 20 µm thick aluminum and copper foil confined with a glass (BK7) substrate. These foils are irradiated by 10 ns, 532 nm laser pulses of energy 25 – 200 mJ corresponding to the intensity range 0.2 – 10 GW/cm2. The plasma temperature at the glass-foil interface is observed to play an important role in the coupling of laser energy to the foil. From our experiments and 1D hydrodynamic simulations, we confirm that moving the glass-foil interface away from the focal plane led to (a) enhanced absorption of the laser beam by the foil resulting in ~ 30 % higher blow-off shock velocities (b) significant changes in the material ejection in terms of increased blow-off mass of the foil (c) lower plasma density and temperatures. The material ejection as well as blow-off shock velocity is higher for Al compared to Cu. The simulated shock evolution in ambient air shows a reasonably good agreement with the experimental results.

We present an acoustic detection technique to study the interaction of two shock waves emitted by two nearby, simultaneous, laser-induced air-breakdown events that resembles the phenomenon of interaction of fluids. A microphone is employed to detect the acoustic shock wave (ASW) from the interaction zone. The experiments were done at various separation distances between the two plasma sources. The incident laser energy of the sources is varied from 25 to 100 mJ in ratios from 1:1 to 1:4. The peak sound pressure of the ASW was compared between the single and dual plasma sources, showing that the pressures are higher for the dual plasma source than that of the single plasma. The evolution of peak sound pressures is observed to depend on (a) the pulse energy of the sources and (b) the plasma separation distance, d. For the equal energy sources, the peak sound pressures increased linearly up to a certain plasma separation distance d, beyond which the pressures saturated and decayed. For the case of unequal energy sources, the peak sound pressures showed an interesting response of increase, saturation, decay, and further increase with plasma separation distance d. These observations indicate the dynamics of acoustic wave interactions across the interaction zone of the two sources depend on the input laser pulse energy as well as the plasma separation distance d.

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