Demonstration of low-mode shape control in indirect-drive double shell implosions at the NIF

Double shell inertial confinement fusion utilizes dense metal shells to compress deuterium-tritium (DT) fuel to fusion conditions. The goal of this campaign is to achieve a volumetric burn as radiation losses from the DT fuel are trapped by the opaque high-Z shell. Volumetric burn involves an alternative path to ignition and exhibits different power balance, matter/energy transport, and mix characteristics in comparison to hot-spot ignition. Our point design double shell uses a volume of liquid DT surrounded by W, Be, foam, and Al shells, and is shot in an indirect drive configuration at the National Ignition Facility (NIF). The overall performance of double shell implosions relies on efficient collisional transfer of kinetic energy between shells, which requires approximate spherical symmetry of the shells during the implosion. Asymmetries may be seeded by hohlraum radiation drive as well as double-shell-specific capsule fabrication artifacts, such as the outer shell assembly joint or shell offsets.

The focus of our current work is on understanding shape transfer between shells as well as on determining the extent to which their symmetry can be controlled. Radiographs from initial experiments using targets with unfilled W inner shells (i.e., no DT fill) generally show evidence of low-mode asymmetries in both inner and outer shells. Hydrodynamic simulations suggest that inner and outer cone beam energies can be altered to correct these low-mode asymmetries. Comparison between experimental data and analogous hydrodynamic simulations was used to design a beam configuration that resulted in an approximately symmetric double shell implosion. Inner vs. outer cone beam energies were varied to achieve < 3% P₂ in both inner and outer shells. This symmetric implosion will be used as the point design for future double shell experiments that seek to evaluate the effects of additional perturbations to target and laser pulse components in order to determine an optimal design.