Larry R. Foreman

The development and advantages of beryllium capsules for the National Ignition Facility

Capsules with beryllium ablators have long been considered as alternatives to plastic for the National Ignition Facility laser ; now the superior performance of beryllium is becoming well substantiated. Beryllium capsules have the advantages of relative insensitivity to instability growth, low opacity, high tensile strength, and high thermal conductivity. 3-D calculation with the HYDRA code NTIS Document No. DE-96004569 (M. M. Marinak et.al. in UCRL-LR-105821-95-3) confirm 2-D LASNEX U. B. Zimmerman and W. L. Kruer, Comments Plasmas Phys. Controlled Thermonucl. Fusion, 2, 51(2975) results that particular beryllium capsule designs are several times less sensitive than the CH point design to instability growth from DT ice roughness. These capsule designs contain more ablator mass and leave some beryllium unablated at ignition. By adjusting the level of copper dopant, the unablated mass can increase or decrease, with a corresponding decrease or increase in sensitivity to perturbations. A plastic capsule with the same ablator mass as the beryllium and leaving the same unablated mass also shows this reduced perturbation sensitivity. Beryllium`s low opacity permits the creation of 250 eV capsule designs. Its high tensile strength allows it to contain DT fuel at room temperature. Its high thermal conductivity simplifies cryogenic fielding.

Review of indirect-drive ignition design options for the National Ignition Facility

Several inertial confinement fusion (ICF) capsule designs have been proposed as possible candidates for achieving ignition by indirect drive on the National Ignition Facility (NIF) laser [Paisner et al., Laser Focus World 30, 75 (1994)]. This article reviews these designs, their predicted performance using one-, two-, and three-dimensional numerical simulations, and their fabricability. Recent design work at a peak x-ray drive temperature of 250 eV with either 900 or 1300 kJ total laser energy confirms earlier capsule performance estimates [Lindl, Phys. Plasmas 2, 3933 (1995)] that were based on hydrodynamic stability arguments. These simulations at 250 eV and others at the nominal 300 eV drive show that capsules having either copper doped beryllium (Be+Cu) or polyimide (C22H10N2O4) ablators have favorable implosion stability and material fabrication properties. Prototypes of capsules using these ablator materials are being constructed using several techniques: brazing together machined hemishells (Be+Cu)...

Metallization of poly(4-methyl-1-pentene) microcellular foam

Low density microcellular foam from poly(4‐methyl‐1‐pentene) (PMP) has been metallized by laser‐induced chemical vapor deposition (LICVD). KrF excimer laser radiation (248 nm) was used to photolytically decompose molybdenum hexacarbonyl with and without a buffer gas. Metal oxycarbide deposits 100 μm in diameter were formed. The microstructure of the PMP foam was unaltered. The deposition was confirmed to be 100 μm using scanning electron microscopy and energy dispersive x‐ray spectroscopy. X‐ray fluorescence was used to determine that approximately 80 ng of material was deposited.

Beta-Layering of Solid Deuterium-Tritium in a Spherical Polycarbonate Shell

In this paper, the authors examine two of the variables that affect the beta-layering process in which nonuniform layers of solid deuterium-tritium (DT) are driven toward uniformity by beta-decay induced sublimation. For these experiments, a 9 mm diameter polycarbonate sphere was partially filled with a 50-50 mix of DT liquid, frozen, and then held at 17 K. The authors measured the equilibration time constant r as functions of solid layer thickness, He exchange gas pressure, and age. Solid layer thicknesses ranged from 200 {mu}m to 650 {mu}, exchange gas pressures from 0 to 600 torr, and age from 0 to 104 days. Results show a significant final solid layer anisotropy with exchange gas pressures above 5 torr, and r values that increased with age by 0.01 min/day for 200 {mu}m-thick layers, and by 0.5 min/day for 650 {mu}m-thick layers. The time constant is shown to be a weak function of exchange gas pressure.

High-resolution optical measurements of surface roughness for beta-layered deuterium-tritium solid inside a re-entrant copper cylinder

A high-resolution optical imaging system and custom-designed image analysis software are used to make surface roughness measurements for deuterium-tritium (D-T) solid layers, equilibrated inside a 2-mm-inside-diameter re-entrant copper cylinder. Several experiments are performed that yield D-T layer thicknesses of between 75 and 139 {mu}m, with equilibration temperatures between 17.4 and 18.8 K. A 1024- x 1024-pixel charge-coupled-device imaging camera, coupled with a Maksutov-Cassegrain long-range microscope, produces a 2.5-{mu}m (single-pixel) image resolution. The error function fitting of the image analysis data produces submicron resolution of the layer interior surface finish. The length scale for the cylinder inner bore is just over 6 mm, and the final layer surface roughness for this length ranges from 3- to 1.7-{mu}m root-mean-square. The feasibility is being explored of using these highly uniform and smooth D-T solid layers inside future targets for inertial confinement fusion reactors to produce surface finishes that will meet target design requirements for the National Ignition Facility. Techniques for improving the D-T solid layer surface finish are examined, limitations of the current D-T cell configuration and fuel mix are discussed, and cell configurations for future experiments are described. 10 refs., 8 figs.

Fabrication of ICF reactor targets based on symmetrization of solid fuel

Tritium self-heats at a rate of 0.97 W/mole because of its ..beta.. activity. This energy drives preferential sublimation of solid T/sub 2/ or DT from regions where a layer is thicker and warmer to regions where the layer is thinner and colder. A frozen tritium layer inside an isothermal container approaches isotropy exponentially with a time constant as short as 14.4 minutes. Targets Isotropized by Radioactively Induced Sublimation (IRIS) and sized for an ICF reactor demonstrate high gain and offer real fabrication advantages. 9 refs., 2 figs.

High resolution optical measurements of beta‐layering in D‐T

An optical method is developed for the study of beta‐layering of solid deuterium‐tritium target capsules for the National Inertial Confinement Program. High‐resolution image data show that the surface roughness do not exceed 2 micrometers 50 hours after first target freezing. (AIP)

Machining sub-millimeter beryllium and aluminum components of laser targets

We show the development of tooling, miniature boring tools, and the machining steps required in the machining operations for sub-millimeter beryllium and aluminum components of laser targets. The targets were built for the Helen Laser at AWE, Aldermaston in the UK and were designed to measure the response of aluminum to the passage of mega-bar shock waves. 1 refs., 8 figs.

Surface roughness measurements of beta-layered solid deuterium-tritium in toroidal geometries

New experiments in a NIF-scale toroidal cylinder have resulted in true shadowgraphs of the DT ice surface. The spectral analysis of the images summed over l-modes 2 through 256 reveal that the surface roughness reaches values just below 1.0 {micro}m at temperatures of 19 K and above. Summing only modes l {ge} 10, the partial surface roughness is below 0.7 {micro}m at 19.5 K. These results indicate that native beta-layering will be sufficient to meet the NIF requirements for DT ice surface finish for both Be and CH ablating shells. The toroidal cylinder incorporates a linear heater along the cylindrical axis to test the concept of surface enhancement due to heat assisted beta-layering in DT. Additionally, with the use of this heater it is possible to symmetrize a pure D{sub 2} layer.

Hohlraum manufacture for inertial confinement fusion

Hohlraums are an integral part of indirect drive targets for Inertial Confinement Fusion (ICF) research. Hohlraums are made by an electroforming process that combines elements of micromachining and coating technology. The authors describe how these target elements are made and extensions of the method that allow fabrication of other, more complex target components.