Instability growth mitigation in double -shell planar experiments using graded density layer and bilayer inner shell

The double-shell platform is a volume ignition concept that takes advantage of low convergence volumetric burn (inner shell CR ~10) at the expense of reduced gain using a high-Z (W) metal pusher. Though low convergence implosions are expected to be more stable, the high Atwood numbers at the foam-pusher interface in double-shell designs makes them susceptible to instability growth that can feed through the pusher causing high-Z material mix into the fuel. Thus, stabilizing the fuel-pusher interface is crucial to achieve a stable double-shell implosion. Typically, a low-Z (Be) tamper layer on top of the high-Z pusher is used to reduce the abrupt jump in Atwood number and reduce instability growth. A graded density layer, where the density gradually increases from Be density (facing the foam cushion) to full W density (facing the DT fuel) is expected to suppress the instability growth more effectively than a Be/W bilayer due to the gradually smoothed Atwood number. We have tested this idea at the OMEGA laser facility using a planar experiment with a halfraum drive. The results show that both the bilayer and graded layer are effective in mitigating a pre-imposed 35um wavelength perturbation. However, the results are inconclusive between the bilayer and graded layer for the 35um wavelength perturbation.