Mitigating laser-imprint effects in direct-drive inertial confinement fusion implosions with an above-critical-density foam layer

Low-density foams of low-/mid-Z materials have been previously proposed to mitigate laser imprint for direct-drive inertial confinement fusion (ICF). For foam densities above the critical density of the drive laser, the mechanism of laser-imprint mitigation relies on the reduced growth rate of Rayleigh–Taylor instability because of the increased ablation velocity and density scale length at the ablation surface. Experimental demonstration of this concept has been limited so far to planar-target geometry. The impact of foams on spherical implosions has not yet been explored in experiments. To examine the viability of using an above-critical-density foam layer to mitigate laser-imprint effects in direct-drive ICF implosions on OMEGA, we have performed a series of 2-D DRACO simulations with state-of-the-art physics models, including nonlocal thermal transport, cross-beam energy transfer, and first-principles equation-of-state tables. The simulation results indicate that a 40-μm-thick CH or SiO2 foam layer with a density of ρ = 40 mg/cm3 added to a D2-filled polystyrene (CH) capsule can significantly improve the moderate-adiabat (α ≈ 3) implosion performance. In comparison to the standard CH target implosion, an increase in neutron yield by a factor of 4 to 8 and the recovery of 1-D compression ρR are predicted by DRACO simulations for a foam-target surface roughness of σrms ≤ 0.5 μm. These encouraging results could readily facilitate experimental demonstrations of laser-imprint mitigation with an above-critical-density foam layer.