General Atomics-Carbon Hydrogen (GA-CH) and General Atomics-Carbon Deuterium (GA-CD) aerogels have applications as inertial confinement fusion (ICF) targets at the National Ignition Facility, Omega Laser Facility, and Z Pulsed Power Facility. However, fusion experiments at these facilities require the fabrication of precise geometries of aerogels, achievable only by machining. Unfortunately, machining low-density (<50 mg/cm(3)) GA-CH aerogels is difficult, given their fragile structure. Higher-density GA-CH aerogels, although easier to machine, are left with a small nub after machining. This work investigates filling the GA-CH gel pores with wax to increase their machinability. The wax was added by exchanging the solvent of the GA-CH gel with melted wax. In addition, 1- to 2-mm spherical voids were created within the aerogels using fused quartz beads that were leached with hydrofluoric acid. Samples were characterized for contaminants, structural damage, dopant loss, and surface roughness using size measurements, scanning electron microscopy, Fourier transform infrared spectroscopy, micro-computed tomography imaging, and optical profilometry. Through advances in aerogel fabrication techniques, progress is made toward testing new ICF target designs.

Author(s): Kritcher, Andrea L; Swift, Damian C; D¨oppner, Tilo; Bachman, Benjamin; Benedict, Lorin X; Collins, Gilbert W; DuBois, Jonathan L; Elsner, Fred; Fontaine, Gilles; Gaffney, Jim A; Hamel, Sebastien; Jenei, Amy; Johnson, Walter R; Kostinski, Natalie; Kraus, Dominik; MacDonald, Mike; Maddox, Brian; Martin, Madison E; Neumayer, Paul; Nikroo, Abbas; Nilsen, Joseph; Remington, Bruce A; Saumon, Didier; Sterne, Phillip A; Sweet, Wendi; Correa Tedesco, Alfredo A; Whitley, Heather D; Falcone, Roger W; Glenzer, Siegfried H

White dwarfs represent the final state of evolution for most stars1–3. Certain classes of white dwarfs pulsate4,5, leading to observable brightness variations, and analysis of these variations with theoretical stellar models probes their internal structure. Modelling of these pulsating stars provides stringent tests of white dwarf models and a detailed picture of the outcome of the late stages of stellar evolution6. However, the high-energy-density states that exist in white dwarfs are extremely difficult to reach and to measure in the laboratory, so theoretical predictions are largely untested at these conditions. Here we report measurements of the relationship between pressure and density along the principal shock Hugoniot (equations describing the state of the sample material before and after the passage of the shock derived from conservation laws) of hydrocarbon to within five per cent. The observed maximum compressibility is consistent with theoretical models that include detailed electronic structure. This is relevant for the equation of state of matter at pressures ranging from 100 million to 450 million atmospheres, where the understanding of white dwarf physics is sensitive to the equation of state and where models differ considerably. The measurements test these equation-of-state relations that are used in the modelling of white dwarfs and inertial confinement fusion experiments7,8, and we predict an increase in compressibility due to ionization of the inner-core orbitals of carbon. We also find that a detailed treatment of the electronic structure and the electron degeneracy pressure is required to capture the measured shape of the pressure–density evolution for hydrocarbon before peak compression. Our results illuminate the equation of state of the white dwarf envelope (the region surrounding the stellar core that contains partially ionized and partially degenerate non-ideal plasmas), which is a weak link in the constitutive physics informing the structure and evolution of white dwarf stars9. Researchers have measured the equation of state of hydrocarbon in a high-density regime, which is necessary for accurate modelling of the oscillations of white dwarf stars.

As opposed to a single shell, the double shell target design provides an alternative implosion platform for high energy density (HED) experiments at the National Ignition Facility (NIF) and related laboratories. The inner shell of this target scheme incorporates a density graded layer to suppress inhomogeneity induced Rayleigh-Taylor instabilities during the implosion. Here we report our efforts to fabricate density graded layers for use as the inner shell of a double shell target using magnetron sputtering as well as the characterization of these layers. Cold welding (spherical targets sticking to either each other or the deposition pan) was observed for many of metals surveyed and seems to be correlated with material ductility. A W-Be gradient layer was successfully fabricated as an inner shell. This material combination is advantageous due to the constituent's large inherent density difference. Microstructural changes are revealed with varying composition including a previously unknown amorphous phase.

The authors present an evaporative initiated chemical vapor deposition (iCVD) coater and use it to establish a submicron bonding process for millimeter‐scale foils with potentially rough surface features. The coater uses a simple benchtop design suited to research labs, with pre‐heated metal pins instead of hot filaments, and direct evaporation of reactants within the chamber. Coatings of poly(glycidyl methacrylate) (pGMA) with thickness 100–800 nm are achieved at rates of 10–40 nm min −1 on substrates common in high energy laser compression experiments. Coating uniformities of 10–30 nm mm −1 are demonstrated in a ≈60 × 10 mm zone under the heated pins. As an aside, the authors further show the ability to coat intentionally non‐uniform layers in a monomer vapor diffusion gradient. Coatings are formed on both plastics and solids ranging from smooth, non‐burred silicon or lithium fluoride to rough and burred metals (aluminum and copper). These coated substrates are then chemically bonded under mild heat and pressure. Detailed surface, thickness, and cross‐sectional characterization is performed to confirm a submicron bond gap and to troubleshoot the common clearance issues from burrs, roughness, and surface curvature. Peeling and dropping bond strength tests confirm the bonds are robust, when coated and assembled under conditions to mitigate clearance issues.

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 mu m thick, 2.8 mm long, 33 mg/cm(3)) 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/cm(3) 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).

In this paper, we describe the reasoning that leads us to focus on the so-called curing process where a solid poly(a-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.

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.

Long superconductor fibers have been continuously produced by electrophoretically depositing REBa2Cu3O7−x (where RE=Y or a selected rare-earth element) powder onto a metal substrate fiber and sintering, then electrophoretically 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 m long and multifilamentary wire 70 m long have been produced.