Hydrodynamic Simulations and Soft X-Ray Laser Interferometric Studies of Energy Transport in Tightly Focused Laser-Heated Aluminum Plasmas

A plasma generated by a tightly focused ~ 14 μm (FWHM), 600 ps duration laser beam at an irradiance of 1013 W cm-2 on a flat aluminum target is investigated. We report new findings that give a better understanding of the energy transport mechanisms in the measured plasma. The formation of a small, on-axis dip or concave electron density profile is observed. Detailed modeling of the spatial and temporal profile of a laser-produced plasma with the 2-D LASNEX hydrodynamics code gives good agreement with the observed features. The observed cold plasma formation along the target surface hundreds of microns away from the small focal spot is generated by heated material outside of the laser focus. Plasma generated by low intensity wings in the laser spatial profile is augmented by soft x-ray radiation from the hot coronal plasma heated by the laser. The simulations show that the x-ray heating will produce a plasma outside the focal spot without the low intensity wings. Strong thermal electron conductive heating due to large thermal gradients continues to generate ablation outside the laser spot and formation of lobes on each side of the focal spot.