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Understanding of the thermonuclear burn in an inertial confinement fusion implosion requires knowledge of the local deuterium–tritium (DT) fuel density. Neutron imaging of the core now provides this previously unavailable information. Two types of neutron images are required. The first is an image of the primary 14-MeV neutrons produced by the D + T fusion reaction. The second is an image of the 14-MeV neutrons that leave the implosion hot spot and are downscattered to lower energy by elastic and inelastic collisions in the fuel. These neutrons are measured by gating the detector to record the 6–12 MeV neutrons. Using the reconstructed primary image as a nonuniform source, a set of linear equations is derived that describes the contribution of each voxel of the DT fuel region to a pixel in the downscattered image. Using the measured intensity of the 14-MeV neutrons and downscattered images, the set of equations is solved for the density distribution in the fuel region. The method is validated against test problems and simulations of high-yield implosions. The calculated DT density distribution from one experiment is presented. [ABSTRACT FROM AUTHOR]

The performance of modern laser-driven inertial confinement fusion (ICF) experiments is degraded by contamination of the deuterium–tritium (DT) fuel with high-Z material during compression. Simulations suggest that this mix can be described by the ion temperature distribution of the implosion, given that such contaminants deviate in temperature from the surrounding DT plasma. However, existing neutron time-of-flight (nTOF) diagnostics only measure the spatially integrated ion temperature. This paper describes the techniques and forward modeling used to develop a novel diagnostic imaging system to measure the spatially resolved ion temperature of an ICF implosion for the first time. The technique combines methods in neutron imaging and nTOF diagnostics to measure the ion temperature along one spatial dimension at yields currently achievable on the OMEGA laser. A detailed forward model of the source and imaging system was developed to guide instrument design. The model leverages neutron imaging reconstruction algorithms, radiation hydrodynamics and Monte Carlo simulations, optical ray tracing, and more. The results of the forward model agree with the data collected on OMEGA using the completed diagnostic. The analysis of the experimental data is still ongoing and will be discussed in a separate publication. [ABSTRACT FROM AUTHOR]

Contact and projection electron radiography of static targets was demonstrated using a laser–plasma accelerator driven by a kilojoule, picosecond-class laser as a source of relativistic electrons with an average energy of 20 MeV. Objects with areal densities as high as 7.7 g/cm2 were probed in materials ranging from plastic to tungsten, and radiographs with resolution as good as 90 μm were produced. The effects of electric fields produced by the laser ablation of the radiography objects were observed and are well described by an analytic expression relating imaging magnification change to electric-field strength. [ABSTRACT FROM AUTHOR]

Neutron, gamma-ray, and x-ray imaging are important diagnostic tools at the National Ignition Facility (NIF) for measuring the two-dimensional (2D) size and shape of the neutron producing region, for probing the remaining ablator and measuring the extent of the DT plasmas during the stagnation phase of Inertial Confinement Fusion implosions. Due to the difficulty and expense of building these imagers, at most only a few two-dimensional projections images will be available to reconstruct the three-dimensional (3D) sources. In this paper, we present a technique that has been developed for the 3D reconstruction of neutron, gamma-ray, and x-ray sources from a minimal number of 2D projections using spherical harmonics decomposition. We present the detailed algorithms used for this characterization and the results of reconstructed sources from experimental neutron and x-ray data collected at OMEGA and NIF. [ABSTRACT FROM AUTHOR]

Emerging approaches to short-pulse laser-driven neutron production offer a possible gateway to compact, low cost, and intense broad spectrum sources for a wide variety of applications. They are based on energetic ions, driven by an intense short-pulse laser, interacting with a converter material to produce neutrons via breakup and nuclear reactions. Recent experiments performed with the high-contrast laser at the Trident laser facility of Los Alamos National Laboratory have demonstrated a laser-driven ion acceleration mechanism operating in the regime of relativistic transparency, featuring a volumetric laser-plasma interaction. This mechanism is distinct from previously studied ones that accelerate ions at the laser-target surface. The Trident experiments produced an intense beam of deuterons with an energy distribution extending above 100MeV. This deuteron beam, when directed at a beryllium converter, produces a forward-directed neutron beam with ~5 ? 109 n/sr, in a single laser shot, primarily due to deuteron breakup. The neutron beam has a pulse duration on the order of a few nanoseconds with an energy distribution extending from a few hundreds of keV to almost 80MeV. For the experiments on neutron-source spot-size measurements, our gated neutron imager was setup to select neutrons in the energy range of 2.5-35 MeV. The spot size of neutron emission at the converter was measured by two different imaging techniques, using a knife-edge and a penumbral aperture, in two different experimental campaigns. The neutronsource spot size is measured 1mm for both experiments. The measurements and analysis reported here give a spatial characterization for this type of neutron source for the first time. In addition, the forward modeling performed provides an empirical estimate of the spatial characteristics of the deuteron ion-beam. These experimental observations, taken together, provide essential yet unique data to benchmark and verify theoretical work into the basic acceleration mechanism, which remains an ongoing challenge. [ABSTRACT FROM AUTHOR]

The neutron imaging system at the National Ignition Facility is an important diagnostic tool for measuring the two-dimensional size and shape of the source of neutrons produced in the burning deuterium-tritium plasma during the stagnation phase of inertial confinement fusion implosions. Very few two-dimensional projections of neutron images are available to reconstruct the threedimensional neutron source. In this paper, we present a technique that has been developed for the 3D reconstruction of neutron and x-ray sources from a minimal number of 2D projections. We present the detailed algorithms used for this characterization and the results of reconstructed sources from experimental data collected at Omega. [ABSTRACT FROM AUTHOR]

To achieve hotspot ignition, an inertial confinement fusion implosion must achieve high hotspot pressure that is inertially confined by a dense shell of DT fuel. This requires a symmetric implosion having high in-flight shell velocity and high areal density at stagnation. The size of the driver and scale of the capsule required can be minimized by maintaining a high efficiency of energy coupling from the imploding shell to the hotspot. Significant 3D low mode asymmetries, however, are commonly observed in indirect-drive implosions and reduce the coupling of shell kinetic energy to the hotspot. To better quantify the magnitudes and impacts of shell density asymmetries, we have developed new analysis techniques and analytic models [Hurricane et al., Phys. Plasmas 27(6), 062704 (2020)]. To build confidence in the underlying data, we have also developed an analytic neutron transport model to cross-compare two independent measurements of asymmetry, which shows excellent agreement across shots for mode-1 (ℓ = 1). This work also demonstrates that asymmetry can introduce potential sampling bias into down-scattered ratio measurements causing the solid-angle-average and uncertainty-weighted-average down-scattered ratios to differ significantly. Diagnosing asymmetries beyond mode-1 (ℓ > 1) presents significant challenges. Using new diagnostic instruments and analysis techniques, however, evidence of significant Legendre mode P2 (ℓ = 2, m = 0) and additional 3D asymmetries (ℓ > 1, m ≠ 0) are beginning to emerge from the high precision activation diagnostic data (real-time nuclear activation detectors) and down-scattered neutron imaging data. [ABSTRACT FROM AUTHOR]

Inertial confinement fusion capsule implosions produce neutron, gamma-ray, and x-ray emission, which are recorded by a variety of detectors, both time integrated and time resolved, to determine the performance of the implosion. Two-dimensional emission images from multiple directions can now be combined to infer three-dimensional structures in the implosion, such as the distribution of thermonuclear fuel density, carbon ablator, and impurities. Because of the cost and complexity of the imaging systems, however, only a few measurements can be made, so reconstructions of the source must be made from a limited number of views. Here, a cylindrical-harmonics decomposition technique to reconstruct the three-dimensional object from two views in the same symmetry plane is presented. In the limit of zero order, this method recovers the Abel inversion method. The detailed algorithms used for this characterization and the resulting reconstructed neutron source from an experiment collected at the National Ignition Facility are presented. [ABSTRACT FROM AUTHOR]

First time-integrated neutron images of a deuterium gas filled capsule were obtained using arrival time gating with the Neutron Imaging System at the National Ignition Facility. Images exist from DT (deuterium and tritium mixture) filled capsules in several energy bands but only at the Omega laser had DD (pure deuterium) filled capsules been imaged. A composite image was derived from an assembly of multiple penumbral neutron images using an iterative Maximum Likelihood reconstruction technique. This was compared with a simulated image from a radiation-hydrodynamic calculation. The observed image size, and shape agree, as do the primary DD, secondary DT neutron yields, and the burn duration. However, the observed cross-sectional profiles, although smaller in half width, extend outside the calculated, suggesting that deuterium has mixed outward into the carbon ablator. The observed X-ray image size (61 μm) is larger than the observed neutron image (51 μm). The calculations also reflect this. X-ray brightness includes carbon as well as deuterium emission. A bright spot, “meteor,” in the X-ray image is seen to move in time-gated images, but is not evident in the neutron image. It does not appear to degrade the neutron yield. [ABSTRACT FROM AUTHOR]

The installation of a neutron imaging diagnostic with a polar view at the National Ignition Facility (NIF) required design of a new aperture, an extended pinhole array (PHA). This PHA is different from the pinhole array for the existing equatorial system due to significant changes in the alignment and recording systems. The complex set of component requirements, as well as significant space constraints in its intended location, makes the design of this aperture challenging. In addition, lessons learned from development of prior apertures mandate careful aperture metrology prior to first use. This paper discusses the PHA requirements, constraints, and the final design. The PHA design is complex due to size constraints, machining precision, assembly tolerances, and design requirements. When fully assembled, the aperture is a 15 mm × 15 mm × 200 mm tungsten and gold assembly. The PHA body is made from 2 layers of tungsten and 11 layers of gold. The gold layers include 4 layers containing penumbral openings, 4 layers containing pinholes and 3 spacer layers. In total, there are 64 individual, triangular pinholes with a field of view (FOV) of 200 μm and 6 penumbral apertures. Each pinhole is pointed to a slightly different location in the target plane, making the effective FOV of this PHA a 700 μm square in the target plane. The large FOV of the PHA reduces the alignment requirements both for the PHA and the target, allowing for alignment with a laser tracking system at NIF. [ABSTRACT FROM AUTHOR]

X-ray and neutrons are commonly used to image inertial confinement fusion implosions, providing key diagnostic information on the fuel assembly of burning deuterium-tritium (DT) fuel. The x-ray and neutron data provided are complementary as the production of neutrons and x-rays occurs from different physical processes, but typically these two images are collected from different views with no opportunity for co-registration of the two images. Neutrons are produced where the DT fusion fuel is burning; X-rays are produced in regions corresponding to high temperatures. Processes such as mix of ablator material into the hotspot can result in increased x-ray production and decreased neutron production but can only be confidently observed if the two images are collected along the same line of sight and co-registered. To allow direct comparison of x-ray and neutron data, a combined neutron x-ray imaging system has been tested at Omega and installed at the National Ignition Facility to collect an x-ray image along the currently installed neutron imaging line of sight. This system is described, and initial results are presented along with prospects for definitive coregistration of the images. [ABSTRACT FROM AUTHOR]

Neutron and x-ray imaging provide critical information about the geometry and hydrodynamics of inertial confinement fusion implosions. However, existing diagnostics at Omega and the National Ignition Facility (NIF) cannot produce images in both neutrons and x-rays along the same line of sight. This leads to difficulty comparing these images, which capture different parts of the plasma geometry, for the asymmetric implosions seen in present experiments. Further, even when opposing port neutron and x-ray images are available, they use different detectors and cannot provide positive information about the relative positions of the neutron and x-ray sources. A technique has been demonstrated on implosions at Omega that can capture x-ray images along the same line of sight as the neutron images. The technique is described, and data from a set of experiments are presented, along with a discussion of techniques for coregistration of the various images. It is concluded that the technique is viable and could provide valuable information if implemented on NIF in the near future. [ABSTRACT FROM AUTHOR]

The neutron imaging system at the National Ignition Facility (NIF) is an important diagnostic tool for measuring the two-dimensional size and shape of the neutrons produced in the burning deuteriumtritium plasma during the stagnation stage of inertial confinement fusion implosions. Since the neutron source is small (~100 µm) and neutrons are deeply penetrating (>3 cm) in all materials, the apertures used to achieve the desired 10-µm resolution are 20-cm long, triangular tapers machined in gold foils. These gold foils are stacked to form an array of 20 apertures for pinhole imaging and three apertures for penumbral imaging. These apertures must be precisely aligned to accurately place the field of view of each aperture at the design location, or the location of the field of view for each aperture must be measured. In this paper we present a new technique that has been developed for the measurement and characterization of the precise location of each aperture in the array. We present the detailed algorithms used for this characterization and the results of reconstructed sources from inertial confinement fusion implosion experiments at NIF. [ABSTRACT FROM AUTHOR]

Neutron and x-ray images are collected at the National Ignition Facility (NIF) to measure the size and shape of inertial confinement fusion implosions. The x-ray images provide a measure of the size and shape of the hot region of the deuterium-tritium fuel while the neutron images provide a measure of the size and shape of the burning plasma. Although these two types of images are collected simultaneously, they are not collected along the same line of sight (LOS). One 14 MeV neutron image is collected on the NIF equator, and two x-ray images are collected along the polar axis and nearly perpendicular to the neutron imaging line of sight on the equator. Both measurements use pinhole apertures to form the images, but existing x-ray imaging provides time-resolved measurements while the neutron images are time-integrated. Detailed comparisons of the x-ray and neutron images can provide information on the fuel assembly, but these studies have been limited because the implosions are not azimuthally symmetric and the images are collected along different LOS. We have developed a conceptual design of a time-integrated x-ray imaging system that could be added to the existing neutron imaging LOS. This new system would allow these detailed studies, providing important information on the fuel assembly of future implosions. Here we present this conceptual design and the expected performance characteristics. [ABSTRACT FROM AUTHOR]

The neutron imaging system at the National Ignition Facility (NIF) is an important diagnostic tool for measuring the two-dimensional size and shape of the neutrons produced in the burning deuterium-tritium plasma during the ignition stage of inertial confinement fusion (ICF) implosions at NIF. Since the neutron source is small (~100 μm) and neutrons are deeply penetrating (>3 cm) in all materials, the apertures used to achieve the desired 10-μm resolution are 20-cm long, single-sided tapers in gold. These apertures, which have triangular cross sections, produce distortions in the image, and the extended nature of the pinhole results in a non-stationary or spatially varying point spread function across the pinhole field of view. In this work, we have used iterative Maximum Likelihood techniques to remove the non-stationary distortions introduced by the aperture to reconstruct the underlying neutron source distributions. We present the detailed algorithms used for these reconstructions, the stopping criteria used and reconstructed sources from data collected at NIF with a discussion of the neutron imaging performance in light of other diagnostics. [ABSTRACT FROM AUTHOR]

First results from the analysis of neutron image data collected on implosions of cryogenically layered deuterium-tritium capsules during the 2011-2012 National Ignition Campaign are reported. The data span a variety of experimental designs aimed at increasing the stagnation pressure of the central hotspot and areal density of the surrounding fuel assembly. Images of neutrons produced by deuterium-tritium fusion reactions in the hotspot are presented, as well as images of neutrons that scatter in the surrounding dense fuel assembly. The image data are compared with 1D and 2D model predictions, and consistency checked using other diagnostic data. The results indicate that the size of the fusing hotspot is consistent with the model predictions, as well as other imaging data, while the overall size of the fuel assembly, inferred from the scattered neutron images, is systematically smaller than models' prediction. Preliminary studies indicate these differences are consistent with a significant fraction (20%-25%) of the initial deuterium-tritium fuel mass outside the compact fuel assembly, due either to low mode mass asymmetry or high mode 3D mix effects at the ablator-ice interface. [ABSTRACT FROM AUTHOR]

Numerical modeling of the neutron imaging system for the National Ignition Facility (NIF), forward from calculated target neutron emission to a camera image, will guide both the reduction of data and the future development of the system. Located 28 m from target chamber center, the system can produce two images at different neutron energies by gating on neutron arrival time. The brighter image, using neutrons near 14 MeV, reflects the size and symmetry of the implosion 'hot spot.' A second image in scattered neutrons, 10-12 MeV, reflects the size and symmetry of colder, denser fuel, but with only ∼1%-7% of the neutrons. A misalignment of the pinhole assembly up to ±175 μm is covered by a set of 37 subapertures with different pointings. The model includes the variability of the pinhole point spread function across the field of view. Omega experiments provided absolute calibration, scintillator spatial broadening, and the level of residual light in the down-scattered image from the primary neutrons. Application of the model to light decay measurements of EJ399, BC422, BCF99-55, Xylene, DPAC-30, and Liquid A suggests that DPAC-30 and Liquid A would be preferred over the BCF99-55 scintillator chosen for the first NIF system, if they could be fabricated into detectors with sufficient resolution. [ABSTRACT FROM AUTHOR]

The neutron imaging diagnostic at the National Ignition Facility has been operating since 2011 generating neutron images of deuterium-tritium (DT) implosions at peak compression. The current design features a scintillating fiber array, which allows for high imaging resolution to discern small-scale structure within the implosion. In recent years, it has become clear that additional neutron imaging systems need to be constructed in order to provide 3D reconstructions of the DT source and these additional views need to be on a shorter line of sight. As a result, there has been increased effort to identify new image collection techniques that improve upon imaging resolution for these next generation neutron imaging systems, such as monolithic deuterated scintillators. This work details measurements performed at the Weapons Neutron Research Facility at Los Alamos National Laboratory that compares the radiographic abilities of the fiber scintillator with a monolithic scintillator, which may be featured in a future short line of sight neutron imaging systems. [ABSTRACT FROM AUTHOR]

Both neutron images and spectra will diagnose ignition implosions at the National Ignition Facility. From the integrated Hohlraum and capsule calculations of copper-doped beryllium capsules using ∼1 MJ of laser energy we have postprocessed neutron spectra and both energy-gated and time integrated neutron images displaying the observable consequences of two-dimensional Hohlraum asymmetries. If low signal precludes multiple down scattered images, we suggest a 6–12 MeV gate. With asymmetries, it is noted that the neutron yield, spectra, and images vary with observation direction. The yield varies only several percent when observed at different angles. Since most asymmetries are expected about the Hohlraum axis, a perpendicular view has the highest priority. The next most informative view would be along the Hohlraum axis, but may be precluded by target chamber structures. We present images at the available port angles and discuss their utility. To facilitate detailed diagnostic simulations with real pinhole geometries or penumbral apertures, we offer a compact disk containing neutron spectra and gated images from various integrated calculations. [ABSTRACT FROM AUTHOR]

The Neutron Imaging System at NIF uses an array of plastic scintillator fibers in conjunction with a time-gated imaging system to form an image of the neutron emission from the imploded capsule. By gating on neutrons that have scattered from the 14.1 MeV DT energy to lower energy ranges, an image of the dense, cold fuel around the hotspot is also obtained. An unmoderated spallation neutron beamline at the Weapons Neutron Research facility at Los Alamos was used in conjunction with a time-gated imaging system to measure the yield of a scintillating fiber array over several energy bands ranging from 1 to 15 MeV. The results and comparison to simulation are presented. [ABSTRACT FROM AUTHOR]

The concepts and initial development efforts for a spatially resolved ion temperature diagnostic are described. The diagnostic is intended for Inertial Confinement Fusion experiments at the National Ignition Facility and is an integration of neutron aperture imaging and ion temperature techniques. The neutron imaging technique is extended by recording tomographic projections of the radiation-to-light converter on a streak camera. The streak record is used to calculate images at multiple times during the arrival of the thermally broadened 14.1 MeV neutron flux. The resulting set of images is used to determine the spatially resolved ion temperature. [ABSTRACT FROM AUTHOR]

High energy electrons have been used to investigate an extension of transmission electron microscopy. This technique, transmission high energy electron microscopy (THEEM), provides two additional capabilities to electron microscopy. First, high energy electrons are more penetrating than low energy electrons, and thus, they are able to image through thicker samples. Second, the accelerating mode of a radio-frequency linear accelerator provides fast exposures, down to 1 ps, which are ideal for flash radiography, making THEEM well suited to study the evolution of fast material processes under dynamic conditions. Initial investigations with static objects and during material processing have been performed to investigate the capabilities of this technique. [ABSTRACT FROM AUTHOR]

A correction to the article "Neutron Source Reconstruction From Pinhole Imaging at National Ignition Facility," published in previous issue, is presented.

Inertial confinement fusion experiments at the National Ignition Facility are designed to understand the basic principles of creating self-sustaining fusion reactions by laser driven compression of deuterium-tritium (DT) filled cryogenic plastic capsules. The neutron imaging diagnostic provides information on the distribution of the central fusion reaction region and the surrounding DT fuel by observing neutron images in two different energy bands for primary (13-17 MeV) and down-scattered (6-12 MeV) neutrons. From this, the final shape and size of the compressed capsule can be estimated and the symmetry of the compression can be inferred. These experiments provide small sources with high yield neutron flux. An aperture design that includes an array of pinholes and penumbral apertures has provided the opportunity to image the same source with two different techniques. This allows for an evaluation of these different aperture designs and reconstruction algorithms. [ABSTRACT FROM AUTHOR]

Recent neutron imaging of experiments at the National Ignition Facility has provided useful information about the hotspot shape and cold-fuel distribution and has also given insight into avenues for improvement. Neutron image reconstruction depends on accurate pointing information because the point-spread function of the neutron aperture is not shift invariant. Current pointing techniques are limited in their accuracy and rely upon detailed information about the as-built structure of the array, which is difficult to determine. We present a technique for extracting high-precision pointing information from both neutron and x-ray images, and a new aperture design with features to facilitate this technique, and allow future co-registration of neutron and x-ray images. [ABSTRACT FROM AUTHOR]