Analysis of surface structures of a metal target from multi-pulse KrF laser ablation

Summary form only given, as follows. Aluminum targets were ablated by focusing a KrF excimer laser (248 nm, 40 ns, <1.2 J) down to a spot size of 0.05 cm/sup 2/ with a fluence of approximately 4.9 J/cm/sup 2/. After a few tens of pulses, surface irregularities (corrugations and pits) progressively emerge, with size 1-100 /spl mu/m which is much larger than the laser wavelength. Such large scale surface roughness causes multiple reflections of the laser light, and may increase the absorption coefficient over a pristine, flat surface by an order of magnitude. Thus, as much as 16% (at room temperature) of the power of the KrF laser may be absorbed by the aluminum target. Scaling laws on the enhanced absorption due to surface roughness are derived. We have also examined various physical mechanisms that lead to these large scale surface structures. The most promising candidate appears to be hydrodynamic instabilities of intense plasma formation near to the surface. A model is developed which yields the growth rate as a function of wave number, thickness of molten layer, energy density and spatial extent of the surface plasma, and the thermophysical properties of the irradiated material. We found that there is a threshold of plasma energy density for the occurrence of the instability.