C. A. Frederick

Fabrication and attachment of polyimide fill tubes to plastic NIF capsules

Abstract We have developed a technique for drawing commercially available polyimide tubing to the required fill tube dimensions. The tubes are then precisely cut with an Excimer laser to produce a clean, flat tip. We have also demonstrated that one can use the Excimer laser to drill less than a 5 μm diameter through hole in the ~150 wall of a NIF dimension GDP shell, and can then create a 10-15 μm diameter, 20-40 μm deep counterbore centered on the through hole with the same laser. Using a home built assembly station the tube is carefully inserted into the counterbore and glued in place with UV-cure epoxy, using a LED UV source to avoid heating the joint. We expect that the same joining technique can be used for Be shells.

Controlling the Pore Size and Gelation Time of Resorcinol Formaldehyde Foam for Fabrication of Direct Drive Targets for ICF Experiments

Abstract Resorcinol Formaldehyde (R/F) foam has been used in the fabrication of direct drive shell targets for Inertial Fusion Confinement (ICF) experiments at the University of Rochester’s Laboratory for Laser Energetics (LLE). Recent cryogenic experiments at LLE using R/F shells have shown the necessity of larger pore foam compared to the standard R/F formulation. In this paper, we report controlling the pore size of R/F foam with concomitant control of the gelation time, which is crucial for successful shell fabrication. The “standard” formulation, with pores of <100 nm, was modified by decreasing the base catalyst to resorcinol concentration ratio creating a large pore R/F foam (∼ >0.5 μm) through reaction limited aggregation. However, this formulation decreased the gelation time, which decreased the yield of shells with proper wall uniformity (∼ 30%) to an unacceptable level of <1%. We developed a technique to achieve control over the gelation time, while keeping the large pore characteristics of R/F to improve shell non-uniformity and increasing the yield to an acceptable level. We also developed a new technique for large pore formation involving changes to the acid catalyst concentration. The effects of this new formulation on the wall uniformity of shells are discussed. The pore distributions obtained using these new R/F foams were characterized using a variety of techniques, including electron microscopy, nitrogen gas adsorption, visible spectroscopy, and small angle x-ray scattering and compared to the standard small pore formulation.

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.

Improving the Yield of Target Quality OMEGA Size PAMS Mandrels by Modifying Emulsion Components

Abstract Poly(α-methylstyrene) (PAMS) shells are made by microencapsulation and used in the fabrication of a large variety of targets for the inertial confinement fusion (ICF) program. Although this process has previously been developed into production mode, the yield of shells with acceptable sphericity and wall uniformity in the OMEGA size range (800–1000 μm) has been poor (∼ 18%). We have made improvements in the yield of these shells by modifying the composition of the outer water solution (W2) in the microencapsulation emulsion. This improvement was achieved by increasing the concentration of Poly Vinyl Alcohol (PVA) from 0.3% to 1.0% and an addition of 0.1% Poly Acrylic Acid (PAA). These modifications were aimed at increasing the interfacial surface tension in the emulsion but also appear to have played a role in density matching the components in the PAMS emulsion. These modifications improved the out of round (OOR) and non-concentricity (NC) of the PAMS mandrels resulting in as increase in the yield of target quality batches based on these basic criteria from 18% to over 80%. Meanwhile, the vacuole content and the surface finish of the PAMS shells were not adversely affected by these changes.

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 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.

Characterization of Embedded Spheres in Opaque Foams

Abstract This paper concerns the methods that were used to build an imbedded sphere in foam target for use on Omega to test theories of astrophysical jets. The core of the target is comprised of a titanium slab that is driven through a titanium washer into a low-density foam with an imbedded sphere. The critical dimension that needed to be known was the location of the center of the sphere with respect to the drive region. Initially, attempts were made to fabricate the sphere imbedded foam precisely, however the foam changed dimensionally during the drying phase of fabrication. The dimensional changes observed were often as large as the specified tolerances, so the foams required post fabrication characterization. Optical characterization of the foams weren’t accurate enough and radiography was required for precision characterization. Once characterized, the sphere needed to be placed in the specified target geometry correct to an accuracy of ±25 μm. The radiography images were imported into a CAD program and these images were used to assemble the target precisely. The methods used provided a well-characterized target with a good build.

EMBEDDING SAPPHIRE SPHERES IN RESORCINOL FORMALDEHYDE AEROGEL FOR ASTROPHYSICAL JET EXPERIMENTS

Abstract Single and double sapphire spheres have been precisely suspended in 300 mg/cm3 RF aerogels without a supporting stalk by modifying parameters such as gelation time and stability of the aerogel precursor solution and use of a vacuum holder for the sphere placement. A process for fabricating a single-sphere target was also developed using a spider silk support (approximately) 100 times thinner than the earlier work on similar targets. Characterization of the ball placement and the aerogel was done by radiography. Data from the characterization of these targets showed that the ball was accurately placed and the aerogel matrix was not significantly altered.

FABRICATION OF MULTIPLE FILL TUBE TARGETS FOR SANDIA NATIONAL LABORATORY

Abstract Target design for the National Ignition Facility requires either a glass or polyimide (PI) fill tube. To study the hydrodynamic effects that are introduced by a fill tube during capsule implosion, fill tube targets were fabricated for experiments at the Z-Pinch facility. Three and four fill tube targets were designed and fabricated to maximize data during each experiment. Targets were made with PI and glass fill tubes on the same capsule to study the shadowing differences between glass and plastic fill tubes. Four tube targets were fabricated with diameters ranging from 10-45 μm to study the effect diameter has on implosion characteristics. Capsules were coated with a germanium-doped layer of glow discharge polymer. Blind holes were drilled in the capsules using an excimer laser. Fill tubes were fabricated using modified capillary pullers and assembly was done on a specially designed assembly station designed for fill tube fabrication. Targets were characterized by optical microscopy and by micron resolution x-ray tomography.