Three-Dimensional moDeling of neuTron-BaseD DiagnosTics To infer Plasma conDiTions

LLE Review, Volume 150 100 Introduction In direct-drive cryogenic inertial confinement fusion (ICF)1 implosions, a target comprising a shell of cryogenic deuterium–tritium (DT) fuel enclosing a vapor region is irradiated using multiple nearly identical laser beams. As the kinetic energy of the imploding shell is converted to the thermal energy of the hot spot, the shell undergoes deceleration and conditions relevant for achieving fusion reactions are obtained. Conditions achieved in the compressed core, at the time of peak fusion neutron production, from a typical cryogenic direct-drive implosion on the OMEGA Laser System2 are shown in Fig. 150.54. The profile was obtained from a spherically symmetric simulation using the code LILAC;3 nonuniformity was ignored in this calculation. Ideally, a hot core is surrounded by a high-density shell, although multidimensional simulations indicate that while this is largely true, the neutron-producing region is typically not centered and the high-density shell can be significantly perturbed. Perturbations are typically quantified in terms of spherical harmonics (from 3-D simulations) or Legendre modes (in 2-D experimental images or 2-D simulations). The goal of cryogenic implosions on OMEGA is to understand the physics of directly driven ICF implosions. This requires observations that could potentially shed light on failure mechanisms including the role of long-wavelength asymmetries on target performance. Three-dimensional (3-D) direct-drive simulations that include the effect of long-wavelength asymmetries performed with the arbitrary Lagrangian–Eulerian (ALE) code HYDRA4 indicate that significant long-wavelength asymmetries corresponding to spherical harmonics up to 4 should be present during the time of neutron production. These asymmetries can be seeded by power imbalance among the 60 beams of OMEGA; by beam mistiming, beam mispointing, or nonuniformities on the inside of the cryogenic layer at the ice–vapor interface; or by the initial error in the placement of the target relative to the center of the target chamber, etc. Longwavelength asymmetries can compromise performance by reducing the clean volume over which neutrons are produced. In Fig. 150.54, the clean volume has a radius of +20 nm. The Rayleigh–Taylor5 growth of the nonuniformities during the deceleration phase of the imploding capsule results in growth at approximately this radius; spikes of the high-density shell penetrate the hot spot, whereas bubbles of lower-density material distort the high-density shell. These large bubbles in the high-density shell may permit heat and fuel to escape, thereby decreasing fusion yields. These asymmetries result in angular variations in areal density, defined as