Dynamics of Collinear Counterpropagating Laser Plasmas under Ambient Atmospheric Conditions

A numerical study of interaction of two counter propagating laser induced atmospheric air plasmas separated by a distance of 4 mm using FLASH ¹ 2D radiation hydrodynamic code is presented. The plasma dynamics and the subsequent shock wave expansion were validated with the experimental results show a good agreement. Second harmonic laser pulses from Q-switched Nd:YAG laser (TITAN-5, 5J, 10 ns, 5 Hz, M/s. Amplitude Technologies) were used to create two plasma sources (S 1 & S 2 ) due to 50 and 100 mJ pulse energies, respectively. The interaction of the two freely expanding plasmas is initiated at $0.9\ \mu \mathrm{s}$ , leading to the formation of soft stagnation layer at the interaction zone. During the interaction of two plasmas a maximum mass density ( $4.7\times 10^{-3}$ gm/cc) and pressure (2.2 MPa) were observed at stagnation layer over a time duration of $1.1\ \mu\mathrm{s}$ . This density and pressure are more than the sum of the individual sources before interaction. A density drag is observed in the source S 1 beyond 1.1 µs because of the penetration of the SW from high energy source (S 2 ) leading to formation of a plasma jetlet. The spatio-temporal evolution of the plasma, shockwave and plasma jetlet dynamics were discussed in terms of the plasma density (n e ), temperature (T e ) and the pressure of individual plasma sources leading to the evolution of plasma jetlet. These colliding plasma plumes are useful to understand the shockwave - plasma interaction ² .