Temporal dependence of the mass-ablation rate in uv-laser-irradiated spherical targets

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Abstract
In this paper we present new measurements of thermal transport in spherical geometry using time-resolved x-ray spectroscopy. We determine the time dependence of the mass-ablation rate by following the progress of the ablation surface through thin layers of material embedded at various depths below the surface of the target. These measurements, made with 6 and 12 uv (351 nm) beams from the OMEGA laser system of the Laboratory for Laser Energetics of the University of Rochester, are compared to previous thermal transport data and to detailed hydrodynamic code simulations. We find agreement with code results for the scaling of the mass-ablation rate with absorbed intensity, but measure larger absolute values than predicted. This finding is interpreted as being the result of irradiation nonuniformities on target. The sharp decrease in the mass-ablation rate after the peak of the laser pulse predicted in simulations is consistent with experimental observations.