Surface instability of multipulse laser ablation on a metallic target

Large scale wavelike patterns are observed on an aluminum surface after it is ablated by a series of KrF laser pulses (248 nm, 40 ns, 5 J/cm(2)). These surface structures have a wavelength on the order of 30 mu m, much longer than the laser wavelength. We postulate that these wave patterns are caused by the Kelvin-Helmholtz instability at the interface between the molten aluminum and the plasma plume. A parametric study is given in terms of the molten layer's thickness and of the spatial extent and kinetic energy density in the laser-produced plasma plume. Also included is an estimate of the cumulative growth in a multipulse laser ablation experiment. These estimates indicate that the Kelvin-Helmholtz instability is a viable mechanism for the formation of the large scale structures. (C) 1998 American Institute of Physics.