Shock ignition: a brief overview and progress in the design of robust targets

Shock ignition is a laser direct-drive inertial confinement fusion (ICF) scheme in which the stages of compression and hot spot formation are partly separated. The fuel is first imploded at a lower velocity than in conventional ICF, reducing the threats due to Rayleigh-Taylor instability (RTI). Close to stagnation, an intense laser spike drives a strong converging shock, which contributes to hot spot formation. This paper starts with a brief overview of theoretical studies, target design and experimental results on shock ignition. The second part of the paper illustrates original work aiming at the design of robust targets and computation of the relevant gain curves. Following Chang et al. [Phys. Rev. Lett. 104 135002 (2010)] a safety factor for high gain, IFT* (analogous to the ignition threshold factor ITF [Clark et al., Phys. Plasmas 15, 056305 (2008)]) , is evaluated by means of parametric one-dimensional simulations with artificially reduced reactivity. SI designs scaled as in Atzeni et al. [New J. Phys. 15, 045004 (2013)] are found to have nearly the same ITF*. For a given target, such ITF* increases with implosion velocity and laser spike power. A gain curve with a prescribed ITF* can then be simply generated by upscaling a reference target with that value of ITF*. An interesting option is scaling in size by reducing the implosion velocity to keep the ratio of implosion velocity to self-ignition velocity constant. At given total laser energy, targets with higher ITF* are driven to higher implosion velocity and achieve somewhat lower gain. However, 1D gain higher than 100 is achieved at (incident) energy below 1 MJ, implosion velocity below 300 km/s, and peak incident power below 400 TW. Two-dimensional simulations of mispositioned targets show that targets with higher ITF* indeed tolerate larger displacements.