Peter L. Gobby

Nuclear diagnostics for the National Ignition Facility (invited)

The National Ignition Facility (NIF), currently under construction at the Lawrence Livermore National Laboratory, will provide unprecedented opportunities for the use of nuclear diagnostics in inertial confinement fusion experiments. The completed facility will provide 2 MJ of laser energy for driving targets, compared to the approximately 40 kJ that was available on Nova and the approximately 30 kJ available on Omega. Ignited NIF targets are anticipated to produce up to 1019 DT neutrons. In addition to a basic set of nuclear diagnostics based on previous experience, these higher NIF yields are expected to allow innovative nuclear diagnostic techniques to be utilized, such as neutron imaging, recoil proton techniques, and gamma-ray-based reaction history measurements.

Development of a neutron imaging diagnostic for inertial confinement fusion experiments

Pinhole imaging of the neutron production in laser-driven inertial confinement fusion experiments can provide important information about the performance of various capsule designs. This requires the development of systems capable of spatial resolutions on the order of 5 μm or less for source strengths of 1015 and greater. We have initiated a program which will lead to the achievement of such a system to be employed at the National Ignition Facility (NIF) facility. Calculated neutron output distributions for various capsule designs will be presented to illustrate the information which can be gained from neutron imaging and to demonstrate the requirements for a useful system. We will describe the lines-of-sight available at NIF for neutron imaging and explain how these can be utilized to reach the required parameters for neutron imaging. We will describe initial development work to be carried out at the Omega facility and the path which will lead to systems to be implemented at NIF. Beginning this year, pr...

Observation of reduced beam deflection using smoothed beams in gas-filled hohlraum symmetry experiments at Nova

Execution and modeling of drive symmetry experiments in gas-filled hohlraums have been pursued to provide both a better understanding of radiation symmetry in such hohlraums and to verify the accuracy of the design tools which are used to predict target performance for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)]. In this paper, the results of a series of drive symmetry experiments using gas-filled hohlraums at the Nova laser facility [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] at Lawrence Livermore National Laboratory are presented. A very important element of these experiments was the use of kineform phase plates (KPP) to smooth the Nova beams. The effect of smoothing the ten Nova beams with KPP phase plates is to remove most of the beam bending which had been observed previously, leaving a residual bending of only 1.5°, equivalent to a 35 μm pointing offset at the hohlraum wall. The results show that the symmetry variation with pointing of implosions in gas-filled ho...

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...

Moderate-convergence inertial confinement fusion implosions in tetrahedral hohlraums at Omega

A highly uniform thermal x-radiation field for indirect-drive inertial confinement fusion implosions may be obtained by irradiating a four-hole, tetrahedral geometry, spherical hohlraum with all 60 Omega laser beams. Implosion studies and calculations [J. M. Wallace et al., Phys. Rev. Lett. 82, 3807 (1999)] indicate a drive uniformity comparable to that expected for the National Ignition Facility [J. A. Painser et al., Laser Focus World 30, 75 (1994)]. With 60 beams distributed over the cavity wall, tetrahedral hohlraums have a natural insensitivity to power balance and pointing errors. Standard, smooth Nova capsules imploded with this drive indicate that moderate convergence-ratio implosions, Cr∼18, have measured-neutron yield to calculated-clean-one-dimensional-neutronyield ratios similar to those previously investigated using the comparatively poor drive uniformity of Nova cylindrical hohlraums. This may indicate that a nonsymmetry-related neutron yield degradation mechanism, e.g., hydrodynamic mixing ...

Indirect drive experiments utilizing multiple beam cones in cylindrical hohlraums on OMEGA

Current plans for time-dependent control of flux asymmetry in the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, and M. Sorem, “The National Ignition Facility Project,” ICF Quart. 5, 110 (1995)] hohlraums rely on multiple beam cones with different laser power temporal profiles in each cone. Experiments with multiple beam cones have begun on the Omega laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester. In addition to allowing symmetry experiments similar to those performed on Nova [A. Hauer et al., Rev. Sci. Instrum. 66, 672 (1995)], the Omega facility allows multiple beam cones to be moved independently to confirm our ability to model the resulting implosion image shapes. Results indicate that hohlraum symmetry behaves similarly with multiple rings of beams as with a single ring, but with the weighted beam spot position used to parametrize the beam pointing.

Tritium Inventory of Inertial Fusion Energy Target Fabrication Facilities: Effect of Foam Density and Consideration of Target Yield of Direct Drive Targets

The tritium inventory of direct drive inertial fusion energy (IFE) target filling facilities is examined in the interest of minimizing the tritium inventory. A model is described that has been developed to evaluate the tritium inventory of the target filling process as a function of filling and layering parameters, as well as target design parameters. Previous studies by A. Nobile et al. showed that the temperature and the fill system void fraction have a significant effect on the tritium inventory. The current study uses the model to examine the effect of deuterium-tritium (DT) ice layering time and density of the CH foam in the target on the tritium inventory. The study shows that increasing the foam density and decreasing the DT ice layering time significantly reduce the tritium inventory. Fortunately, one-dimensional target design calculations indicate that the foam density in the direct drive target can be increased to ~200 mg/cm3 without significant degradation of the target yield. Having evaluated and minimized the theoretical tritium inventory, calculations were performed with more realistic batch filling scenarios. The inventories associated with “real” filling scenarios approach the theoretical minimum inventory as the number of batches is increased, resulting in tritium inventories that seem acceptable for future IFE target DT filling facilities.

Foam-buffered spherical implosions at 527 nm

Creation of a low density, high temperature plasma buffer between the absorption and ablation layers of a directly driven inertial confinement fusion implosion capsule has been proposed as a means to reduce “early time” imprint from laser nonuniformities. This thermal smoothing blanket might be created from a low density foam layer wrapped around the deuterium–tritium filled microballoon. Preliminary spherical implosion tests of this concept using a polystyrene foam layer surrounding a glass microballoon were performed at the Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)], using a 527 nm drive wavelength. Comparison of capsule yield and imploded core symmetry showed promising improvements in overall target performance, relative to one-dimensional undegraded hydrodynamic simulations, when the foam-buffer layer was present.

Single-mode Rayleigh-Taylor growth-rate measurements with the OMEGA laser system

The results from a series of single-mode Rayleigh-Taylor (RT) instability growth experiments performed on the OMEGA laser system using planar targets are reported. Planar targets with imposed mass perturbations were accelerated using five to six 351-nm laser beams overlapped with total intensities up to 2.5×1014 W/cm2. Experiments were performed with both 3-ns ramp and 3-ns flat-topped temporal pulse shapes. The use of distributed phase plates and smoothing by spectral dispersion resulted in a laser-irradiation nonuniformity of 4%–7% over a 600-μm-diam region defined by the 90% intensity contour. The temporal growth of the modulation in optical depth was measured using through-foil radiography and was detected with an x-ray framing camera for CH targets with and without a foam buffer. The growth of both 31-μm and 60-μm wavelength perturbations was found to be in good agreement with ORCHID simulations when the experimental details, including noise, were included. The addition of a 30-mg/cc, 100-μm-thick po...