Deborah J. Clark

Progress toward the development and testing of source reconstruction methods for NIF neutron imaging.

Development of analysis techniques for neutron imaging at the National Ignition Facility is an important and difficult task for the detailed understanding of high-neutron yield inertial confinement fusion implosions. Once developed, these methods must provide accurate images of the hot and cold fuels so that information about the implosion, such as symmetry and areal density, can be extracted. One method under development involves the numerical inversion of the pinhole image using knowledge of neutron transport through the pinhole aperture from Monte Carlo simulations. In this article we present results of source reconstructions based on simulated images that test the methods effectiveness with regard to pinhole misalignment.

The Neutron Imaging Diagnostic at NIF

A neutron imaging diagnostic has recently been commissioned at the National Ignition Facility (NIF). This new system is an important diagnostic tool for inertial fusion studies at the NIF for measuring the size and shape of the burning DT plasma during the ignition stage of Inertial Confinement Fusion (ICF) implosions. The imaging technique utilizes a pinhole neutron aperture, placed between the neutron source and a neutron detector. The detection system measures the two dimensional distribution of neutrons passing through the pinhole. This diagnostic has been designed to collect two images at two times. The long flight path for this diagnostic, 28 m, results in a chromatic separation of the neutrons, allowing the independently timed images to measure the source distribution for two neutron energies. Typically the first image measures the distribution of the 14 MeV neutrons and the second image of the 6-12 MeV neutrons. The combination of these two images has provided data on the size and shape of the burning plasma within the compressed capsule, as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core.A neutron imaging diagnostic has recently been commissioned at the National Ignition Facility (NIF). This new system is an important diagnostic tool for inertial fusion studies at the NIF for measuring the size and shape of the burning DT plasma during the ignition stage of Inertial Confinement Fusion (ICF) implosions. The imaging technique utilizes a pinhole neutron aperture, placed between the neutron source and a neutron detector. The detection system measures the two dimensional distribution of neutrons passing through the pinhole. This diagnostic has been designed to collect two images at two times. The long flight path for this diagnostic, 28 m, results in a chromatic separation of the neutrons, allowing the independently timed images to measure the source distribution for two neutron energies. Typically the first image measures the distribution of the 14 MeV neutrons and the second image of the 6-12 MeV neutrons. The combination of these two images has provided data on the size and shape of the burning plasma within the compressed capsule, as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core.

Investigations into reconstruction techniques for the National Ignition Facility Neutron Imaging System

Neutron imaging is currently being developed as a primary diagnostic for inertial fusion studies at the National Ignition Facility (NIF). It is an attractive diagnostic for measuring asymmetries in the burn region and will be able to operate at neutron fluences found during ignition scale implosions. The most straightforward technique for imaging of the spatial distribution of deuterium-tritium (DT) fusion neutrons utilizes a simple pinhole aperture, which blocks all neutrons outside of the solid angle defined by the pinhole and results in a blurred image at the detector. We are currently investigating source image reconstruction techniques from detector images. Source reconstructions from Monte Carlo neutron transport (MCNP) calculations are shown to emulate hydrodynamic simulations with imposed Legendre asymmetries to high accuracy.

Performance characteristics of the neutron imaging diagnostic at NIF

The neutron imaging diagnostic has recently been commissioned at the National Ignition Facility. We will present the diagnostic performance characteristics, which have been measured with the collection of these first neutron images. The goal for this diagnostic is to collect two pinhole images at two different times. The long flight path results in a chromatic separation of the neutrons, the first image will be of the 14 MeV neutrons and the second image of the 10–12 MeV neutrons. The combination of these two images will provide data on the size and shape of the compressed capsule as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core. The imager uses an array of 20 pinholes and three mini-penumbra machined in 20 cm of layered gold and tungsten, with an apex at 32.5 cm from the source to produce images in a scintillator array at 2800 cm. This geometry provides a magnification factor of 85 at the scintillator. The scintillator is a coherent array of scintillating fibers, which is viewed from the two ends by two fast-gated image collection systems. The first neutron images, collected in February, 2011, have provided the first measure of system performance at NIF. These results will be presented along with an interpretation of future system performance.