Three-Dimensional Reconstructions of Cold Fuel Shells Using Fluence-Compensated Down-Scattered Neutron Imaging at the National Ignition Facility

Understanding the three-dimensional (3D) distribution of dense cold fuel at stagnation is essential for diagnosing performance limitations in inertial confinement fusion (ICF) implosions, particularly as the National Ignition Facility (NIF) achieves higher gains. The Nuclear Imaging System (NIS) provides a critical diagnostic capability, recording both primary and down-scattered neutron images to probe the conditions within imploding targets. Down-scattered neutron images are sensitive to the density and geometry of the cold fuel surrounding the hotspot. Currently, information about the cold fuel distribution is inferred using fluence-compensated down-scattered neutron imaging (FCDSNI), a technique that corrects for the spatial convolution of down-scattered images with the primary neutron fluence. This method yields high-fidelity information about cold fuel structure but is limited to two-dimensional reconstructions from a single line of sight. This work extends FCDSNI to full 3D by combining images from two orthogonal NIS lines of sight and applying cylindrical harmonics decomposition to reconstruct the three-dimensional morphology of the cold fuel shell. The resulting technique offers a new diagnostic perspective on cold fuel asymmetries and structure, providing critical insight into stagnation physics, hotspot performance, and overall ICF implosion behavior.