Investigating Effects of Xenon Capsule Gas Fill Dopant in National Ignition Facility Experiments

Io Kleiser, California Institute of Technology

The National Ignition Facility uses a high-powered laser system to spherically implode a capsule of fuel to temperatures and densities required for hydrogen fusion. The laser is split and aimed from 192 directions into a hohlraum containing the fuel capsule. The laser beams are incident on the hohlraum's inside walls and generate a bath of X-rays, which ablates the capsule's outer shell and causes it to implode. Three-dimensional effects such as diversion from spherical symmetry as well as hydrodynamical instabilities hinder the ability to achieve ignition, but significant progress has been made toward mitigating these effects. Still, the capsule's early physical evolution, even without 3-D complications, is not well understood. Indirect-drive exploding pusher (IDEP) experiments use smaller capsules with thinner shells that better mimic 1-D simulations because smaller factors radially compress the capsules. Xenon dopant can be used as a probe for physics in the capsule interior as it is compressed, but even a small amount of Xenon can change the implosion dynamics. We run a series of 1-D simulations using the radiation-hydrodynamics code HYDRA using various capsule gas fill densities and Xe dopant concentrations to investigate the effects of both parameters on the neutron emission histories, which are a proxy for nuclear reaction rate. We find that the neutron production histories can be fit well with a double Gaussian profile, which roughly represents emission from shock convergence and compression phases of the implosion. Variations in gas density and Xe concentration have predictable effects on the heights, widths and peak times of the neutrino emission. We show dependencies of the neutron production profile as well as capsule convergence ratio on these two parameters, which can later be compared to measurements from IDEP experiments in order to better understand the early evolution of spherical capsule implosions.

Abstract Author(s): I. Kleiser, L.B. Hopkins