Double Shell Plans and First Results from Outer Shell Keyhole Experiments

Double-shells are an alternative approach to achieving indirect drive ignition on NIF. These targets consist of a low-Z ablatively-driven outer shell that impacts a high-Z inner shell filled with DT fuel. In contrast to single-shell designs, double-shell targets burn the fuel via volume ignition, albeit with a lower gain. While double-shell capsules are complicated to fabricate, their design includes several beneficial metrics such as a low convergence pusher (C.R. $<$ 10), low implosion speed (250 km/s), a simple few-ns laser drive in a vacuum hohlraum, less sensitivity to hohlraum asymmetries, and low expected laser-plasma instabilities. We describe plans for developing double shell capsule implosions on NIF, and discuss challenges as well as uncertainties and trade-offs in the physics issues compared to single-shells, such as sensitivity to hard x-ray preheat of the inner shell. First experimental results measuring hard x-ray preheat, shock breakout and shock symmetry from outer-shell experiments using the NIF Keyhole platform will be presented.