Hydrodynamics of Conically Guided Fast Ignition Targets

The fast ignition concept requires the generation of a compact, dense, pure fuel mass accessible to an external ignition source. The current baseline fast ignition target is a shell fitted with a reentrant cone extending to near its center. Conventional direct or indirect drive collapses the shell near the tip of the cone, and then an ultraintense laser pulse focused to the inside cone tip generates high-energy electrons to ignite the dense fuel. Two-dimensional (2-D) calculations of this concept have sparsely explored the large design space available to optimize compaction of the fuel and maintain the integrity of the cone. Experiments have generally validated the modeling while revealing additional complexities. Away from the cone, the shell collapses much as does a conventional implosion, generating a hot, low-density, inner-core plasma that exhausts out toward the tip of the cone. The hot, low-density inner core can impede the compaction of the cold fuel, lowering the implosion\burn efficiency and the gain, and jetting toward the cone tip can affect the cone integrity. Thicker initial fuel layers, lower velocity implosions, and drive asymmetries can lead to decreased efficiency in converting implosion kinetic energy into compression. Fast ignition burn hydrodynamics can generate additional convergence and compression. We describe 2-D and one-dimensional approaches to optimizing designs for cone-guided fast ignition.