Effects of Tamper Thickness on Hydrodynamic Instability Growth in Double Shell Implosions

The double shell is an inertial confinement fusion concept consisting of an outer low-Z ablator and an inner high-Z pusher that contains thermonuclear fuel. The outer shell is driven inwards, colliding with and transferring momentum to the inner shell, which then compresses and heats the fuel. The high-Z pusher reduces radiative losses from the fuel compared to single-shell designs that use more transparent low-Z materials, lowering the temperature required for ignition. If successful, the double shell could provide an alternative path to studying burning plasma in the laboratory. However, controlling hydrodynamic instability growth on the outer surface of the dense pusher during its inward acceleration phase remains a key challenge. Outer surface instability growth can feed through to the inner surface, providing large seeds for deceleration phase growth that rapidly increase the surface area and reduce the efficacy of radiation trapping. Excessive instability growth can also cause the shell to rupture, greatly reducing the compression and heating of the fuel.

We present two-dimensional radiation-hydrodynamics simulations of double shell implosions with imposed surface non-uniformities. We find that performance depends sensitively on the thickness of the low-Z tamper surrounding the high-Z pusher, supporting previous predictions [Milovich et al. (2004) Phys. Plasmas 11, 1552]. The main drive shock radiates ahead of itself, ablating the tamper and launching a radiative precursor shock into it. Thicker tampers delay when this precursor shock impacts the outer surface of the pusher without significantly altering the main shock timing. This reduces the time over which the outer surface is hydrodynamically unstable, but it comes at the cost of increased mass and reduced 1D performance. For thick enough tampers, the main shock catches up to the precursor shock inside of the tamper, resulting in a peak in 2D performance. Different tamper materials (e.g., plastic or Al) require different thicknesses to reach this optimal performance point. We show that increasing the tamper thickness is predicted to improve the robustness of double shell implosions.