Cindy R. Christensen

First results of pinhole neutron imaging for inertial confinement fusion

Results are presented for the first implementation of pinhole imaging of inertial confinement fusion-produced neutrons. Raw images are shown, together with mathematical reconstructions of the source objects, for both spherical and asymmetric implosions. These reconstructions are considerably sharpened with respect to the raw images. They rely on the accurate calculation of the point-spread function, including neutron penetration into the material defining the pinhole. Proton recoil in the scintillator material and irregularity in scintillator fiber packing must be considered. The statistics of the system are inferred, which allows the use of simulations to demonstrate the robustness of the reconstructions to noise.

The influence of asymmetry on mix in direct-drive inertial confinement fusion experiments

The mix of shell material into the fuel of inertial confinement fusion (ICF) implosions is thought to be a major cause of the failure of most ICF experiments to achieve the fusion yield predicted by computer codes. Implosion asymmetry is a simple measurable quantity that is expected to affect the mix. In order to measure the coupling of asymmetry to mix in ICF implosions, we have performed experiments on the OMEGA laser [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)] that vary the energy of each of the sixty beams individually to achieve a given fraction of L2, the second-order Legendre polynomial. Prolate, symmetric, and oblate implosions resulted. Three different fill pressures were used. Simultaneous x-ray and neutron images were obtained. The experiments were modeled with a radiation/hydrodynamics code using the multi-fluid interpenetration mix model of Scannapieco and Cheng. It fits the data well with a single value of its one adjustable parameter (0.07±0.01). This agreement is demonstrated ...

Multifluid interpenetration mixing in directly driven inertial confinement fusion capsule implosions

Mixing between the shell and fuel in directly driven single shell capsule implosions causes changes in yield, burn history, burn temperature, areal density, x-ray image shape, and the presence of atomic mix. Most observations are consistent with a mix model using the same values of its single free parameter as with indirectly driven single shell and double shell capsules. Greater mixing at lower gas pressure fills reduces capsule yield. Time dependent mixing growth causes truncation of the burn history. This emphasizes early yield from the center of the capsule, raising the observed burn temperature. Mixed fuel areal densities are lower because fuel moves through the shell and the observation weights earlier times when areal density is lower. Shell x-ray emission mixing into the fuel fills in the limb brightened image to produce a central peak. Implosions of 3He filled capsules with a layer of deuterated plastic show substantial atomic mix.

Progress on neutron pinhole imaging for inertial confinement fusion experiments

Neutron imaging provides a powerful diagnostic for understanding the performance of inertial confinement fusion ignition capsules and the drive mechanism imploding them. To achieve the spatial resolution and fielding capability needed at the National Ignition Facility requires a staged approach that simultaneously pushes the limits of extant capabilities while developing new techniques that will extend to the National Ignition Facility regime. To this end, new pinhole assemblies have been designed and fabricated using very high-precision machining equipment. These assemblies have been fielded successfully at Laboratory for Laser Energetics, University of Rochester and have provided impetus for new aperture designs and new ideas for detectors, which are now the limiting element in the system resolution.

Goals for and design of a neutron pinhole imaging system for ignition capsules

Neutron yield at the National Ignition Facility (NIF) or the Laser MegaJoule (LMJ) will range from 1019 for a capsule that ignites and burns well to below 1015 for one that fails to ignite. Expected image sizes in deuterium–tritium (DT) neutrons decrease with the neutron yield. At 1018–1019 yields the capsules have ignited and are burning main fuel, producing images with full width at half maximum (FWHM) of ∼100 μm which require a 200 μm field of view and would need 10 μm resolution. Marginally igniting capsules, with yields of 1017 to 1018, burn the hot spot and some main fuel. Their neutron images are smaller, ∼60 μm FWHM, and require ∼120 μm field of view, with 7 μm resolution needed. Below ∼1017, the capsule fails to ignite and a field of view of ∼100 μm suffices to image the hot spot which might be ∼30 μm FWHM with a resolution of ∼5 μm. Images in downscattered neutrons are as large or larger than the time integrated images, have ∼5% of the brightness, and require correspondingly larger fields of vie...

γ-ray 'bang-time' measurements with a gas-Cherenkov detector for inertial-confinement fusion experiments

In a laser driven inertial-confinement fusion experiment, bang time is defined as the time between the laser light impinging the target and the peak of the fusion reactions. Bang time is often used to compare computed predictions to experiment. Large laser facilities, such as NIF and LMJ, which are currently under construction, will produce yields far in excess of any previous inertial-confinement fusion experiment. One of the implications of such high yields is that fusion γ rays, which have branching ratios four orders of magnitude less than that of fusion neutrons, may be used to diagnose bang time. This article describes the first of such γ-ray bang-time measurement made using the OMEGA laser facility at the Laboratory for Laser Energetics, University of Rochester. The diagnostic used for this was a gas Cherenkov detector. The experimental setup, data and error analyses, and suggested improvements are presented.

Backlighter predictive capability

Correctly predicting the intensity and spatial extent of an area backlighter is important in optimizing the design and analysis of a laser-based experiment. In this work, the spatial extent of an area backlighter is calculated using a view factor code to obtain the laser illumination pattern and then converting to x rays using the measured x-ray conversion efficiency. The view factor model can also be compared to a simple illumination calculation. The models were validated with experiments where five 1-ns-square OMEGA [Boehly et al., Opt. Commun. 133, 495 (1997)] laser beams containing a total of 1.85 kJ were directed onto Fe or Ti foils. The predicted emission size was compared to time-gated two-dimensional images of the Fe emission region or to time-integrated images from both Fe and Ti. The models correctly predict the spatial extent of the emitting region for the first hundred picoseconds. The emission region grows logarithmically with time during the laser pulse; eventually reaching a diameter that is 1.6 times the initial laser spot size. Folding the x-ray conversion efficiency into the calculated intensities allows prediction of backlighter brightness and structure that is useful in optimizing the experimental design.

Target Fabrication: A View from the Users

Abstract Targets are used for a variety of purposes, but ultimately we use them to validate codes that help us predict and understand new phenomena or effects. The sophistication and complexity of High Energy Density Physics (HEDP) and Inertial Confinement Fusion (ICF) targets has increased in to match the advances made in modeling complex phenomena. The targets have changed from simple hohlraums, spherical geometries, and planar foils, to 3-dimensional geometries that require precision in construction, alignment, and metrology. Furthermore, material properties, such as surface morphologies and volume texture, have significant impact on the behavior of the targets and must be measured and controlled. In the following we will discuss how experimental physicists view targets and the influence that target construction has on interpreting the experimental results. We review a representative sampling of targets fabricated at the Los Alamos National Laboratory that are used in different experiments in support of ICF and HEDP.

High-speed x-ray imaging in high-power laser experiments

X-ray imaging is one essential tool for capturing phenomena that occur when high-irradiance lasers interact with complex, optically thick targets. We use x-ray backlighting and emission to measure the result of such interactions at experiments on the Omega laser and the Z-machine z-pinch facilities. In this presentation, we will show some of the images collected with a variety of experiments, we will discuss some of the difficulties we overcame, and look to issues that will arise with higher-energy lasers and larger objects.

Fusion γ Ray Signals from ICF Targets Recorded with Streak Camera

Summary form only given. For a dozen years, the fusion reaction rate in inertial confinement fusion (ICF) capsules has been determined by monitoring the production rate of fusion neutrons. Future high-yield targets producing >1015 neutrons should allow reaction rates to be measured with γ rays produced in deuterium-tritium (DT) fusion. Using γ rays makes the measurement independent of target-to-detector distance, but the low branching ratio for γ-ray production (<10-4) makes the task difficult at current ICF facilities where shot yields are <1014 neutrons. Two detector systems based on Cherenkov gas cells have been tested at the OMEGA laser facility. Incident γ rays produce forward-directed, relativistic electrons in a converter foil at the entrances to high-pressure (up to 100 psi) gas cells containing CO2 gas. The electrons generate Cherenkov light when they travel faster than the speed of light through the CO2 gas. Reflective optics collects the light at the output of each cell. For one system the signal is recorded with a fast photomultiplier tube (PMT) at the output of the gas cell. The other system uses relay optics to transfer the Cherenkov light to a streak camera several meters away. The PMT work has been presented in an earlier paper. This paper describes the streak camera based detector system and initial measurements made with it