Schlieren imaging investigation of successive laser-induced breakdowns in air

Successive laser-induced breakdowns were visualized at various energies and time delays using high-speed schlieren imaging at a frame rate of 50 kHz.. Images of single laser-induced breakdown were captured for incident energies of 10 mJ, 21 mJ and 30 mJ from 1 μs up to 1 ms after the pulse. The behavior of the breakdown remained the same with all energies, but the size of the breakdown and the energy absorption increased with increasing energy. The energy absorption of successive laser-induced breakdowns was measured for nine combinations of first and second pulse energies of 10 mJ, 21 mJ, and 30 mJ for time intervals between pulses ranging from 50 ns to 100 μs. Schlieren images were recorded for all nine energy combinations for time intervals of 50 ns and 100 μs. For a time interval of 50 ns, the shockwave from successive laser-induced breakdown had reduced propagation speed and strength when compared with single laser-induced breakdown with the same total energy. At a time interval of 100 μs, turbulent structures were formed resulting from the propagation of the second shockwave though the density region created by the first laser-induced breakdown, with the scale of the turbulent structures decreasing with increasing energy.