Spallation induced by ultrashort laser pulses at critical tension

Spallation, as induced by 500.3em0exfs laser pulses at the maximum possible tension (critical tension) in a 251000-0ptμm aluminum slab, was investigated by means of a one-dimensional fluid code. In the framework of our defect-free fluid model, spallation takes place through spinodal decomposition, a mechanism that differs from the usual mechanism of growth and coalescence of natural defects. Due to the slowing down of the hydrodynamic processes near critical tension, the spinodal decomposition time scale is about 10.3em0exns. Strain rates of about 10⁸0.3em0exs^-1 and spall thicknesses of a few microns are obtained, in agreement with recent experiments using short laser pulses. The critical tension in the simulation (12.80.3em0exGPa) was somewhat larger than the tension inferred from experiments (8.50.3em0exGPa). Because of rapidly decreasing hydrodynamic coupling at higher laser fluences, the required laser spallation threshold fluence as predicted by the code (4100.3em0exJ∕cm²) is far higher than in experiments (250.3em0exJ∕cm²). This large discrepancy in the spallation threshold fluence values might be due either to differences in the mechanisms through which spallation takes place, or to the specific choice of model for the equation of state.