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Direction-dependent asymmetric expansion of laser-induced shockwaves in air

Laser induced breakdown of air is used to create a SW by focusing a 7 ns, 532 nm, 10 Hz Nd:YAG laser. Plasma created at the breakdown launches a compression wave into the nearest ambient air molecular layers that propagate at supersonic velocities. The propagation characteristics of SWs are studied double probe-beam deflectometry method, by two noninteracting probe beams that are parallel to each other and perpendicular to the propagation direction of the SW creating laser beam. The evolution of the laser induced SWs in air created at the focus along the direction of propagation (forward probe, FP) and in the direction opposite to the laser propagation (backward probe, BP) of the breakdown creating laser beam, are studied. The nature of the SW is estimated from the arrival time (τ) measured at different distances (Z) from the focal volume. At an input laser energy of 45 mJ, the arrival time information in the backward probe showed τ ∞ Z2.5 behavior indicating spherical SWs and in the forward direction followed τ ∞ Z1.6 showing planar expansion of the SWs revealing direction dependent asymmetric expansion of SWs across the focal plane. The pressure of the SWs estimated using counter pressure corrected point strong explosion theory along with the experimentally measured shock velocities are used to generate the P - U Hugoniot of shock compressed air.