Double-front plasma produced from high-intensity nanosecond laser ablation of aluminum: Time-resolved imaging and hydrodynamic modeling

An interesting phenomenon of dual fronts was observed experimentally for plasma produced through high-intensity nanosecond (ns) laser ablation of aluminum in a gas environment with a small laser spot, and the underlying mechanisms were revealed using an experimentally validated hydrodynamic model. Time-resolved imaging experiments show that the plasma has two fronts with different shapes and propagation behaviors. Front A propagates rapidly in the early stage with a cone-like shape but becomes almost stationary after the laser pulse ends, and afterward, it resumes obvious propagation with its shape mostly in the form of a small tip. Unlike Front A, Front B propagates much more slowly in the early stage but continues propagating even after the laser pulse ends. Front B maintains a near-hemispherical shape. With the help of the hydrodynamic model, it has been found that Front A is a laser-supported detonation wave, and the dominant propagation-driving mechanism is energy advection followed by laser absorption in the early stage. Front B is found to originate from the shock front in the gas generated due to the expansion of the ablated aluminum vapor. The model simulations reveal that the early-stage high pressure at Front A causes a gas jet moving backward toward the aluminum target and deeply penetrating into the aluminum vapor. Laser conditions required to generate the double-front plasma have been discussed.