Three-dimensional reconstruction of cold fuel shells using down-scattered neutron imaging at the National Ignition Facility

In inertial confinement fusion experiments at the National Ignition Facility, the cold fuel density distribution during peak neutron production indicates the symmetry of imploded laser-driven deuterium–tritium fusion capsules. Since implosion asymmetry can degrade performance, characterizing the three-dimensional structure of the compressed fuel is vital for diagnosing implosion performance. The nuclear imaging system provides two-dimensional imaging of 14 MeV fusion neutrons on three nearly orthogonal lines of sight and 6–12 MeV down-scattered neutrons along two equatorial lines of sight. The 14 MeV images provide the shape of the fusion burn region, and the down-scattered images contain information about the cold fuel distribution, convolved with the fusion burn region. Two approaches are used to infer the cold fuel distribution from these measurements. The first deconvolves the two-dimensional primary fluence from the down-scattered image to provide a two-dimensional fluence-compensated down-scattered image. The second infers a three-dimensional density distribution using the three-dimensional primary emission and the down-scattered images. We compare these methodologies using synthetic multi-mode implosion data and experimental data from igniting experiments, focusing on assumptions, strengths, and limitations. We demonstrate that fluence-compensated images, while computationally efficient and useful for low-mode shape and shell dimensions from a single view, cannot reliably represent complex three-dimensional density structures even with multiple views. The more computationally expensive three-dimensional density reconstructions capture major asymmetries quantitatively in physical units. However, limited viewing angles can create artifacts, indicating that additional lines of sight would improve reconstruction fidelity and enable comprehensive characterization of the fuel and implosion physics.