Measurements of the Effect of Adiabat on Shell Decompression in Direct-Drive Implosions on OMEGA

Measurements of the effect of adiabat $\left( \alpha \right)$ on the shell thickness were performed in direct-drive implosions. The maximum in-flight shell thickness was obtained using a novel technique where the outer and inner surfaces of the shell were simultaneously measured using self-emission images of the imploding target. When reducing the shell's adiabat from $\alpha =6$ to $\alpha =4.5,$ the shell thickness was measured to decrease from $75\mu \mbox{m}$ to $60\mu \mbox{m,}$ but when decreasing the adiabat further $\left( {\alpha =1.8} \right),$ the shell thickness was measured to increase to $75\mu \mbox{m.}$ The measured shell thickness, shell trajectories, neutron bang time, and neutron yield were reproduced by two-dimensional simulations that include laser imprint, nonlocal thermal transport, cross-beam energy transfer, and first-principles equation-of-state models. These results show that the decompression of the shell measured for low-adiabat implosions was a result of laser imprint. Additional information on the evolution of the density profile was obtained using x-ray radiography. The backlighter was created with six of the 60 OMEGA laser beams, with the pointings and energies of other beams adjusted to maintain a uniform implosion. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.