J. F. Hund

IMPROVING THE WALL UNIFORMITY OF RESORCINOL FORMALDEHYDE FOAM SHELLS BY MODIFYING EMULSION COMPONENTS

Abstract Previously we have developed a production process for both standard density (100 mg/cc) and high-density (180-200 mg/cc) resorcinol formaldehyde (RF) foam shells with a triple orifice droplet generator. These foam shells are needed for direct drive inertial confinement laser fusion experiments on the OMEGA laser facility at the University of Rochester. Although this process has been developed into production mode, the yield of high density RF (HDRF) and standard density (SDRF) shells with acceptable wall uniformity has been poor. This yield depends on the type of RF shell that is being fabricated. For HDRF this yield is ~5% while for the SDRF shells the yield is ~30%. We have made improvements in the yield of these shells that meet the wall uniformity specification by modifying the composition of the outer oil solution (O2) in the microencapsulation emulsion. This improvement was achieved by a small addition (0.60 wt.%) of a styrene-butadiene-styrene (SBS) block copolymer into the outer oil (O2) solution that increased the interfacial tension of the emulsion system as well as the viscosity of the O2 solution. This modification improved the out of round and concentricity of the RF foam shells resulting in an increase in the yield of shells that meet the target wall uniformity specifications.

Silica, Metal Oxide, and Doped Aerogel Development for Target Applications

Abstract A variety of silica, metal oxide, and metal doped aerogels are being developed for use as laser target materials. Silica aerogels have been produced with controlled densities as low as 5 mg/cc, and have been produced as bulk molds. Recently, 100 mg/cc small beads and hollow shells have also been fabricated using microencapsulation techniques. Metal oxide aerogels such as tantalum oxide (Ta2O5) and tin oxide (SnO2) are two other low-density materials that have been fabricated. Aerogels with embedded metal particles are also of interest and several methods for producing these composite aerogels are being explored. Each method limits excessive aggregation of the metal so that the end product has a uniform loading of small metal particles. Ion implantation is being investigated as another method that allows more control of the metal doping. With ion implantation the metal dopant can be placed in a narrow distribution beneath the surface of an aerogel, and initial results of 1 MeV Au- implanted in 67 mg/cc SiO2 are described.

Fabrication of Ta2O5 Aerogel Targets for Radiation Transport Experiments Using Thin Film Fabrication and Laser Processing

Abstract Experiments on the Omega laser at the Laboratory for Laser Energetics require tantalum oxide (Ta2O5) aerogel thin films with a thickness ranging from 70 to 150 μm and densities of 250 and 500 mg/cm3. Experiments have been done with the aerogel in a disk geometry with diameters ranging from ~2 to 3 mm with annular slots machined into it and without the slots. These experiments place demanding specifications on the targets in terms of thickness, dimensionality, and mass density variation. Future radiation experiments at the National Ignition Facility will require larger targets ~7 mm in diameter and 200 μm thick with more complex features. In the past these targets have been conventionally machined from a starting billet of aerogel ~5 mm in diameter and height. Through a series of steps the aerogel was eventually machined down to the desired thickness. This was a long and arduous labor-intensive process that had high attrition rates and an overall yield of ~50%. We have improved this process by developing a new fabrication technique involving casting the foam to the desired thickness and then laser processing to create the desired features. This technique yields targets that meet the demanding specifications used in recent experiments while increasing throughput, yield, and available feature complexity in targets.

Fill Tube Assembly Development for Omega and NIF Shell Applications

Abstract A new technique has been developed for attaching glass and polyimide tubes to National Ignition Facility and OMEGA-scale capsules. Resorcinol-formaldehyde foam and CD fill tube assemblies are designed for OMEGA laser capabilities by the University of Rochester Laboratory for Laser Energetics. Because of the porous structure of the foam, test procedures have been implemented to guarantee the target assemblies are leaktight under room and cryogenic temperatures. Based on the type and conditions of the target, ultraviolet glue or two-part epoxy glue is applied to particular sections on the assembly. Development has been conducted to overcome issues dealing with the adhesives, test procedures, and fill tube designs described in this paper.

Development and Fabrication of NIF-Scale Resorcinol Formaldehyde Foam Shells for ICF Experiments

Abstract Possible designs for future direct- and indirect-drive targets required for the National Ignition Facility (NIF) include resorcinol formaldehyde (R/F) foam shells. Depending on the experiment type, the shell dimensional specifications can range from 1.5- to 3.5-mm diameter with a 100- to 200-μm wall. This paper will discuss the adjustments and modifications of the process developed for smaller-diameter (˜900-μm) OMEGA-scale R/F shells that were used to fabricate these higher-aspect-ratio NIF-scale shells.

IMPROVEMENTS ON PERMEATION GDP COATINGS FOR RESORCINOL FORMALDEHYDE FOAM SHELLS FOR CRYOGENIC EXPERIMENTS ON OMEGA

Abstract Resorcinol formaldehyde (RF) foam shells are needed for direct-drive inertial confinement laser fusion experiments at the University of Rochester OMEGA laser facility. As previously reported, the addition of long-chained polymers to the fabrication process has improved shell wall uniformity, but this change has led to a lower yield (from ~40 to ~15%) of shells that are gas retentive after the application of glow discharge polymer (GDP) using the standard deposition technique. We have improved this yield by modifying the coating conditions of the GDP overcoating process by modifying the background coating pressure from the constant 75 mTorr to using a two-step coating process of a high-pressure coating at 250 mTorr followed by low-pressure coating of 75 mTorr. This modification has improved the yield of the gas retention on the styrene-butadiene-styrene RF shells from ~15 to ~60%. We have found that the surface roughness of these shells is also improved from ~45 nm root-mean-square (rms) to ~20 nm rms. This technique, however, leads to a slight shrinkage of shells, which will be described.

Fabrication Capabilities for Spherical Foam Targets Used in ICF Experiments

This paper reviews the processes developed at General Atomics (GA) in the past several years to fabricate a variety of spherical foam targets at various densities for the inertial confinement fusion (ICF) community. The two most common chemical systems used to produce spherical foam targets have been resorcinol-formaldehyde (R/F) aerogel and divinylbenzene (DVB). Spherical targets have been made in the form of shells and beads with diameters ranging from approximately 0.5 to 4.0 mm, and densities from 100 mg/cc to 250 mg/cc, with typical high yield of intact shells or beads of 90%-95%. Permeation barriers have been developed and deposited on both R/F and DVB shells. We have also made R/F foam shells with higher pore size (0.10-0.50 mum) in order to increase the cryo-fill fraction when these shells are cryogenically layered with D2. Another spherical target that is currently under development that will also be discussed is silica aerogels shells and beads. Other foam target materials currently under development, such metal doped R/F aerogel beads for extreme ultra violet (EUV) source experiments will also be discussed

Fabrication of Thin CH and CD Films and Patterned Films Using a Heat Press Technique for the NIF and OMEGA Magnetic Recoil Neutron Spectrometer

Abstract Deuterated polyethylene films as thin as 50 μm have been produced using a four-post heated press method. These films are then cut into the desired shape (foil) and are used for the magnetic recoil spectrometer (MRS). The MRS is a diagnostic tool for inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF) and at the OMEGA Laser Facility at the University of Rochester Laboratory for Laser Energetics. The films produced by this method are more uniform and have fewer defects than foils prepared by previous methods. Whereas traditional films had a thickness limitation of 100 μm, this new heat pressing method allows the fabrication of 50-μm films, which is the desired thickness for MRS foils. The foils produced and characterized by General Atomics have decreased the neutron measurement uncertainty of the MRS instrument by a factor of 2 compared to the previous foils. This work has also been extended to fabricate thin polymer films and films with sinusoidal patterns from other polymeric materials for ICF experiments. This paper will discuss the heat pressing technique used in the fabrication of these films for the NIF and OMEGA MRS as well as other patterned and flat polymer films. The morphology of these foils and the advantages that they provide will also be discussed.

Fabrication and Characterization of Aluminum Oxide Aerogel Backlighter Targets

Abstract Aluminum oxide aerogel can be used as a backlighter target to provide a radiation source for diagnostics during ICF experiments. To demonstrate the feasibility of this type of target, it was necessary to cast thin pieces of aerogel for plasma emission studies of aluminum oxide. We were able to demonstrate density control over a range of 50-400 mg/cc, and, furthermore, cast the aerogel as a thin (0.6-0.7 mm), smooth monolith that did not require additional machining. The fabrication of these targets begins with a solution of aluminum chloride, ethanol, and water and is then catalyzed with propylene oxide to gel within molds to form the shape. Supercritical drying with carbon dioxide provides the dry aerogel. Various target densities were made by adjusting the relative amounts of starting materials and post treatment condition. The finished materials were characterized for density, pore size, and water content. Initial freestanding targets of 98 mg/cc have been fabricated and shot, and other similar targets of densities from 50-400 mg/cc have been fabricated for future experiments.

Improvements to Fill Tube Design for Direct-Drive NIF and Fast Ignition Applications

Abstract Fill tubes are being implemented to meet direct-drive National Ignition Facility (NIF) target designs and eliminate the need for permeation filling of targets. Significant improvements have been made to the fill tube designs for the NIF-scale CD and fast ignition targets to accommodate fuel-layering experiments at the University of Rochester Laboratory for Laser Energetics. The initial fill tube design had a number of issues that contributed to the nonuniformity of the deuterium (D2) ice layer and low fabrication yield of targets. Redesign of the entire target has significantly improved the D2 ice layering by reducing thermal perturbations. These design changes also made a more robust target that can survive the handling required in fabrication and testing. This paper will detail the target design aspects that were altered, including adjusting the fill tube aspect ratio, removing the thermally conductive support stalk, and adding a thermally conductive coating on the fill tube.