Fred Elsner

Development of high quality poly(α-methylstyrene) mandrels for NIF

Abstract Recently, we developed a new method for making spherical poly(α-methylstyrene) (PαMS) mandrels. The process utilizes a small amount (<0.1wt%) of high-molecular-weight poly(acrylic acid) (PAA) in the suspending medium, which substantially increases the interfacial tension during curing relative to methods using poly(vinyl alcohol) (PVA) and yields extremely round capsules. The PAA is also beneficial for centering of the core water, leading to exceptionally concentric capsules. However, fully cured capsules made by this method displayed a significant level of high frequency surface debris that became especially problematic when the mandrels were subsequently overcoated. To solve this problem we examined the use of PAA in conjunction with PVA in order to reduce these surface features, and explored numerous variations of concentration and timing of the PVA addition. The optimum conditions were found to be initial use of PAA for centering and symmetry of the mandrels, followed by removal of the PAA medium, washing of the mandrels with water, and finally transfer to PVA solution for completion of the curing cycle. Glow discharge polymer shells made from these mandrels have power spectra that meet the ignition capsule design requirements.

Fabrication of long length Bi-2223 superconductor tape using continuous electrophoretic deposition on round and flat substrates

We have developed a continuous fabrication process for producing long lengths of Bi-2223 superconductor tapes. The process involves sequentially electrophoretically depositing and sintering superconductor and then silver layers on a substrate, followed by rolling and thermal processing. Both round and flat silver substrates have been used. Bi-2223 tapes made using flat silver substrates require only a few processing steps. Transport critical current densities at 77K in zero applied magnetic field exceeding 20000 A/cm2 have been obtained.

Understanding the Critical Parameters of the PAMS Mandrel Fabrication Process

Abstract In this paper, we describe the reasoning that leads us to focus on the so-called curing process where a solid poly(ɑ-methylstyrene) (PAMS) shell is formed from the initial solution phase. We demonstrate the existence of a percolation zone at about 55 wt% PAMS, beyond which the roundness of the shell can be expected to be irreversible. Using a simple model and a few supporting experiments to account for the rate of mass transfer of the fluorobenzene solvent phase, we show that curing rate is determined almost entirely by just a short exposure, to the sweeping gas, of the shells that graze the free surface of the curing bath as they move around in it. We propose here that specific control of the curing conditions at percolation would enable rounder mandrels.

Zinc Oxide–Coated Poly(HIPE) Annular Liners to Advance Laser Indirect Drive Inertial Confinement Fusion

Abstract Laser indirect drive is hindered, in part, by two problems: “wall motion” resulting from ablation of the hohlraum inner wall and “preheat” of the fuel capsule. To mitigate wall motion and preheat, a mid-Z–coated high internal phase emulsion, poly(HIPE) foam liner (5.7-mm diameter, 150 μm thick, 2.8 mm long, 33 mg/cm3) was developed and integrated into the hohlraum interior. A zinc oxide coating was applied throughout the poly(HIPE) foam using atomic layer deposition to achieve 149 ± 14 mg/cm3 bulk density. Preliminary data collected from actual shots at the National Ignition Facility suggest the inclusion of the poly(HIPE) liner reduced preheat threefold and stimulated Brillouin scattering (SBS) fivefold relative to an existing reference shot on a gold hohlraum (wavelength shift also contributed to SBS reduction).

Fabrication of long length Bi-2223 superconductor tape using a continuous electrophoretic coating process

We have developed a unique fabrication process for producing long lengths of Bi-2223 superconductor tapes. The process uses a continuous electrophoretic coating technique for sequentially depositing superconductor and silver layers on a substrate, by heat treating and rolling steps. The process offers a number of advantages over competing techniques.<<ETX>>

Metrology Feasibility Study in Support of the National Direct-Drive Program

Abstract The 100-Gbar Laser Direct Drive program calls for ablator capsules with no defects larger than 0.5 μm in lateral dimension and fewer than ten defects with lateral dimensions between 0.1 and 0.5 μm. Compared to laser indirect drive capsules, this represents > 10× reduction of defect length scale and >500× reduction in defect number density. This presents major challenges to both fabrication and metrology. In this paper, we will discuss the proof-of-principle work conducted at General Atomics to identify metrology techniques suitable for 100-Gbar target characterization. We present a detailed study of dark-field imaging, laser scatterometry, and environmental scanning electron microscopey. We identify dark-field imaging as the best approach for meeting the 100-Gbar metrology needs.

Innovations in Target Fabrication Techniques at the University of Michigan

Abstract The University of Michigan has been fabricating targets for OMEGA campaigns since 2003. These experiments explore supernova-relevant high-energy-density physics. The complexity of recent target designs has made it necessary to explore new methods of producing components that satisfy experimental needs. Interest in the dynamics of nonaxisymmetric shocks has led to the development of polyimide tubes with noncircular cross sections. For our latest Thomson scattering target, shielding was a very important component to the target design. We employed techniques to bend gold foils, enabling complex geometries without any of the seams inherent when two separate foils are pieced together. Machined acrylic bases are used to support all the components on our targets, contributing further to their repeatability and providing us with a method that eases our build. Here, we present improvements in our techniques, along with our basic tried-and-true methods of producing repeatable targets.

Development of High Temperature Superconductor Coated Metal Fiber and Multifilamentary Wire

Long superconductor fibers have been continuously produced by electrophoretically depositing REBa2Cu3O7−x powder onto a metal substrate fiber and sintering, then eletrophoreticaly depositing silver and sintering. After collecting the coated fiber on a take-up spool, the entire spool is batch-oxygenated to form the 90 K superconducting phase. Multiple fibers are then continuously unspooled and soldered into a copper channel to form the final multifilamentary high temperature superconductor wire. Superconducting fibers over 1000 meters long and multifilamentary wire 450 meters long have been produced.