Simulations of high-gain shock-ignited inertial-confinement-fusion implosionsusing less than 1 MJ of direct KrF-laser energy

摘要

In this paper, we report on recent numerical simulations of inertial-confinement-fusion (ICF) implosionsusing the FAST radiation hydrocode at the U.S. Naval Research Laboratory. Our study focuses on threeclasses of shock-ignited target designs utilizing less than 1 MJ of direct, krypton-fluoride (KrF) laserenergy, which was "zoomed" to maximize the coupling efficiency. In the shock-ignition approach [R.Betti, C.D. Zhou, K.S. Anderson, et al., Phys. Rev. Lett. 98 (2007) 1550011, a moderate-intensity,compressive laser pulse is followed by a short-duration high-intensity spike that launches a spherically-convergent shock wave to ignite a thick shell of compressed fuel. Such an arrangement appears to offerseveral significant advantages, including a low ignition threshold, high gain, and less susceptibility to thedeleterious effects of hydrodynamic and laser-plasma instabilities. According to one-dimensionalsimulations, fusion gains over 200 can be achieved with shock-ignited targets using less than 750 kJ oflaser energy. This represents a significant improvement in performance over conventional centrally-ignited designs. To examine the stability of these targets, several two-dimensional simulations were alsoperformed that incorporated realistic perturbation sources such as laser imprinting and roughnessspectra for inner/outer pellet surfaces. Although the simulations indicate that appreciable low-modedistortion of the fuel shell can occur at late time as a result of these perturbations, high gains are stillachieved in many cases owing to the low in-flight aspect ratios of shock-ignited targets. We shouldremark, though, that the high convergence ratios of these same designs suggest that other sources oflow-mode asymmetries, which were not considered in this study (e.g., beam misalignment and energy-balance errors), may be important in determining overall pellet stability and performance. We discussthese issues, as well as other salient design considerations for shock-ignited ICF targets.