Neutron spectroscopy in laser-direct-drive inertial confinement fusion implosions
Owen Mannion, Laboratory for Laser Energetics
Friday, March 12, 2021
In laser-direct-drive (LDD) inertial confinement fusion implosions, a spherical deuterium–tritium (DT) filled target is directly illuminated and compressed by a laser to generate a low-density (~1 g/cm3), hot (>1-keV) fusing plasma, which is surrounded by a low-temperature (<500-eV), dense (>100-g/cm3) DT fuel layer. These implosions are used in high-energy-density fusion energy science experiments that require a detailed understanding of the hot spot and dense fuel layer conditions near peak compression. Neutron spectroscopy is a key diagnostic technique in these experiments since the primary DT fusion neutron energy spectrum can be used to infer the hot-spot velocity, apparent ion temperature, and fusion yield, while the scattered neutron spectrum can be used to infer the shell areal density, temperature, and mean velocity near peak compression. Measurements of the neutron energy spectrum emitted from LDD implosions on the OMEGA laser are made using a suite of quasi-orthogonal neutron time-of-flight (nTOF) detectors. In this talk, the nTOF detector suite on OMEGA and the analysis techniques used to infer the target conditions from the neutron energy spectrum measurements will be described. Three-dimensional reconstructions of the hot-spot flow velocity, the apparent ion temperature distribution, and the areal-density distribution will be discussed. These 3-D reconstructions are found to be strongly correlated with neutron yield and the asymmetry observed in the x-ray self-emission images of the hot spot. Finally, it will be shown how target performance can be improved by applying laser-drive corrections derived from the hot-spot velocity measurements. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.