Non-spherical symmetry development of underwater shock waves created by laser-induced breakdown

As a controllable alternative to cavitation collapse-induced shock waves, numerous cavitation studies on laser-induced breakdown have been carried out in hydromechanics. When the laser focusing region is not spherical, the shock waves caused by laser breakdown also exhibit non-spherical symmetry propagation. Recently, some researchers have proposed the linear superposition theory based on the far field measurement data to explain this asymmetry, assuming that it is essentially the linear superposition of multiple wave fronts caused by multiple points of laser-induced breakdown that leads to the asymmetric propagation of shock waves. In this study, measurements of shock wave propagation processes with different breakdown energies are carried out based on a nanosecond resolution photogrammetry system, and the propagation velocities of shock waves in different directions are directly measured using a double exposure technique on a single frame. In the experiment, the velocity of the shock wave at the beginning of the breakdown was measured up to nearly 4000 m/s. The early shock wave front was ellipsoidal, and the propagation velocity in the laser incident direction was generally slower than that in the perpendicular direction, decaying to the speed of sound in water within 1000 ns after the breakdown, and the wave front gradually approached to a circle. The variability of the shock wave front pressure ratio in the laser propagation direction and the vertical direction implies that the linear superposition theory applicable to the far field is not applicable to the near field. There may be more complex mechanism for the near-field shock wave propagation process.