Measurements of the non-uniformities seeded by NIF ignition capsule ablator materials

Current NIF ignition target designs contain the DT fuel inside spherical capsules made of either Cu-doped Be or high density C (HDC). Both candidate materials are polycrystalline, and are expected to respond anisotropically to the initial compression wave of the NIF compression sequence. Estimates of the amplitudes of the non-uniformities seeded by each type of ablator suggest that these capsules should remain stable during the subsequent implosion; however, experiments are needed to verify these estimates. We describe experiments designed to measure shock front perturbations induced by the microscopic polycrystalline non-uniformities of these two ablator materials. The measurement method employs a time-resolved two-dimensional imaging VISAR illuminated by a 2 ps laser pulse, which captures spatial variations in the velocity across the shock front transmitted through the ablator. The measurement is carried out over an 800 $\mu $m field of view with relative velocity sensitivity $\Delta $V/V $\sim $ 10$^{-4}$, and over perturbation wavelengths in the range from 3-4 $\mu $m to 50 $\mu $m.