Measurements of Laser-Imprint Mitigation Using an Above-Critical-Density Foam Layer for Direct-Drive Inertial Confinement Fusion*

In laser-direct-drive inertial confinement fusion, laser imprint caused by nonuniformities in the laser drive due to laser speckle can seed the Richtmyer–Meshkov and Rayleigh–Taylor hydrodynamic instabilities, which adversely affect the compression of the imploding shell. Reducing the growth rate of the Rayleigh–Taylor instability caused by laser imprint relies on increasing ablation velocity and density scale length at the ablation surface. To achieve these parameters and mitigate imprint we have proposed layering an above-critical-density foam layer on the surface of the target. Experimental validation of the foam-based imprint mitigation using x-ray radiography in planar experiments will be presented. Foams and solid targets were generated through the two-photon polymerization 3-D printing process and were compared to mass-equivalent polystyrene typically used in direct-drive shells. The targets with foam had a significant reduction in rR modulations (a factor of over 2.5 in rms) and the modulations that were measured were primarily caused by foam structure rather than laser imprint. The data collected from these experiments have allowed for iteration and improvements in target design, which are being used in the creation of 3-D printed spherical targets for the upcoming year.