Measuring and increasing the safety margin of high-gain shock-ignited targets

Shock ignition is a laser direct-drive inertial confinement fusion scheme, in which the stages of compression and hot spot formation are partly separated. The fusion fuel is first imploded at somewhat lower velocity than in conventional schemes, reducing the risks associated to Rayleigh-Taylor instability (RTI). The hot spot is created at the end of the implosion by a converging shock-wave driven by a final spike of the laser pulse. Significant research activity has been devoted to assessing the feasibility of shock ignition. In particular, we studied an all-DT target (the HiPER target), by means of analytical models and 1D and 2D radiationhydrodynamics simulations. In shock ignition, the separation of fuel compression and ignition allows some design flexibility , when targets are up-scaled from a (theoretically) marginally igniting small target to larger dimensions. We determined scaling laws for different scaling options, and computed gain curves by 1D simulations of families of scaled targets [6]. The unavoidable modeling uncertainties (well evidenced, e.g., by the recent NIF experiments indicate that any credible design has to include large safety margins. For high-gain shock ignition we use a 1D safety factor, ITF∗, analogous to the 1D ignition threshold factor, ITF, used to characterize NIF indirect drive targets. In a previous work [10] we computed ITF∗ for the HiPER target, and determined its dependence on implosion velocity and spike power. We then generated gain curves at given ITF∗. In this paper we report further studies, aiming at improving design realism, and at increasing target robustness. In addition to the HiPER target, we consider a target originally proposed by G. Schurtz and the CELIA-Bordeaux group, consisting of a relatively thick DT layer and a plastic ablator. The simulation code DUED has been used in all the simulations reported here.