HiPER project target studies on shock ignition: design principles, modeling, energy scaling, risk reduction options

targets. In previous publications we had identified a window in laser parameter space for the ignition of a simple, small target (with fuel mass of less than 0.3 mg), previously designed for fast ignition. We had also studied aspects of the robustness to parameter deviation from nominal values as well as to hydrodynamic instabilities and non uniform irradiation. Aspects of the scaling with target size had also been discussed. Recent work aimed at improving target and laser modelling (e.g., by considering targets with a more realistic structure, and 3D irradiation schemes) and increasing target robustness. In particular, we show that the separation of the stages of fuel compression and hot spot creation, typical of shock ignition, introduces some design flexibility when scaling targets to higher size. This can be exploited in the future, once limitations set by laser-plasma instabilities and hydrodynamic instabilities will be assessed experimentally. We have determined analytic scaling laws for the different design options, and used detailed numerical simulations to generate the relevant gain curves. We are studying ways to reduce the growth of Rayleigh-Taylor instabilities both at the ablation front and at the hot-cold fuel interface. Concerning irradiation schemes, we have analyzed ways to increase robustness to laser errors and target misplacement; we are also studying polar-drive schemes, which could be used to test shock ignition on the NIF or LMJ lasers, originally designed for indirect-drive laser fusion. In the final part of the presentation we will discuss a few open issues for shock ignition, requiring specific experimental and theoretical efforts.