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AbstractHighly complex targets are constructed by General Atomics for astrophysically relevant experiments conducted by the University of Chicago on the OMEGA laser facility through the National Laser Users’ Facility (NLUF) program.Several novel target components are fabricated, precision assembled, and extensively measured in support of this campaign and have evolved over the last 3 years to improve both the science and assembly. Examples include unique laser-machined polyimide grids to enhance plasma mixing at the target center, precision-micromachined cylindrical shields that also act as component spacers, drawn glass target supports to suspend physics packages at critical distances, and tilted pinholes for collimated proton radiography.Target component fabrication and evolution details for the NLUF Turbulent Dynamo (TDYNO) campaign are presented, along with precision-assembly techniques, metrology methods, and considerations for future TDYNO experiments on OMEGA.

AbstractCapsule fill tube assemblies (CFTAs) consist of an ablator capsule and fill tube via a laser-drilled funnel hole. This hole tapers from 17-μm diameter at the outer surface of the ablator capsule to less than 5-μm diameter on the inside of the capsule over approximately 200 μm of wall thickness. Demand for better understanding of the fill tube perturbation during the capsule implosion has driven advancements in the fill tube design. Engineering efforts have been made on hydrodynamic growth radiography assemblies (HGRs) using multiple tube-design variations, including alternative angles, depths, sizes, and location with engineered defects to showcase fill tube effects during an implosion. Testing has shown that these CFTAs and HGRs have survived all fabrication and transport to and from General Atomics (GA) to Lawrence Livermore National Laboratory. These assemblies have also passed cryogenic testing at GA. An overview of alternative CFTA designs, fabrication methods, and developments is presented.

AbstractCapsules for inertial confinement fusion require precise measurement of isolated features and domes on the capsule’s outer surface. Features that are too large must be removed. A 4pi capsule mapping and characterization system has been developed to map, identify, and measure domes using a Leica confocal microscope. An ultraviolet wavelength laser was integrated to laser-ablate the offending domes that exceed the allowable mix mass. Current process methods to remove domes require three different stations in different locations. The 4pi system achieves automated capsule handling, metrology, and laser polishing/ablation of domes on one device without losing track of the capsule’s orientation. The measurement technique and metrology accuracy are compared to patch atomic force microscopy scans and phase-shifting diffraction interferometer measurements with good correlation. The laser polishing method has demonstrated analogous results to the current process methods, but in an automated fashion. Additionally, the 4pi capsule-handling capability of the system has been used to laser-ablate purposeful engineered designs into specialty capsules.

AbstractEquation-of-state experiments using VISAR require the refractive index of undoped glow discharge polymer (GDP) and Ge-doped GDP at 532-nm-wavelength light. The index was measured with two different techniques. The first technique used measurements of pulsed laser reflections off a GDP foil suspended in refractive index fluid standards. Fluids with different indices were replaced until minimum reflection was achieved; this occurred at the matching index of the fluid and GDP film. The index of the correct matching fluid (or fluid mixture) was measured with an Abbé refractometer to produce nD(the refractive index at sodium D line, 589 nm) and was corrected for wavelength using manufacturer-supplied Cauchy equation coefficients. The second technique used interferometry to measure fringe shift over GDP and Ge-GDP bumps when submerged in various refractive index fluid standards. The fringe shift was minimized when matching the indices of the fluid and film. The refractive indices at 532 nm were 1.563 and 1.570 for undoped GDP and Ge-doped GDP, respectively.

AbstractGeneral Atomics has supported the Los Alamos National Laboratory Defect Implosion Experiment series on OMEGA with the process design and production of banded, gas-tight, glow discharge polymer (GDP) capsules. Production of a banded target is a multistep, multidisciplinary process requiring micromachining for the band, GDP coating for the capsule wall, and aluminum sputter coating to seal the capsules for subsequent gas fill. Challenges included applying a micromachining technique to create the channel that would result in the desired band after coating, and modification of the aluminum coating process to create a permeation barrier that would cover the banded region to allow for gas filling. Information describing the fabrication steps and characterization techniques employed to analyze the banded targets will be presented.

AbstractA thin gold layer is deposited as a liner on the interior of a uranium hohlraum to protect from oxidation of uranium. X-ray fluorescence (XRF) spectrometry was chosen as the liner thickness measurement method for its accuracy, speed, and ease of measurement. The process is noncontact and nondestructive. The thicknesses were determined using a micro-XRF spectrometer unit with analysis software. The accuracy of the measurements was verified against qualified standards. The method was used to accurately measure gold liner thicknesses of cylindrical hohlraums, and it gave initial promising results for measuring the thickness of a boron-doped gold layer when corrected for the gold atom fraction.

AbstractLow-density foam shells are currently being employed as direct drive targets on the Omega laser facility at the University of Rochester. For cryogenic shots, only a thin layer of glow discharge polymer (GDP) is required over these foam shells to hold the D2(or DT) fill provided the capsules are re-filled after cooling. Room temperature surrogate experiments, however, require an additional permeation barrier of aluminum on GDP coated foam shells. This barrier should have a permeation time constant of at least 4 h for D2at room temperature. To study this coating, 0.1 μm layers of Al were deposited via magnetron sputtering onto the surface of GDP shells and GDP coated foam shells. The foam shells were 180 mg/cc resorcinol formaldehyde (RF) with a GDP thickness of 3-5 μm; the GDP shells used for this study had a wall thickness of 25-30 μm. Preliminary data shows that the permeation rate of D2for smooth GDP shells is lower than for GDP coated RF shells with a similar thickness of Al. The main factor in this difference appears to be the surface roughness of the shells.

AbstractFielding a cryogenic ICF capsule for near-term ignition shots requires a fill-tube to deliver fuel to the capsule. Polymer capsules (0.9 and 3.1 mm o.d.) with a monolithic fill-tube have been produced. The fill tube is formed on the capsule by plasma polymer coating on a mandrel with a depolymerizable stalk. We report recent advances achieved in monolithic fill-tube capsule production. The micro-assembly station for attaching PAMS stalk to the mandrels was improved to reduce fillet and fill tube diameters and increase throughput. Excimer laser etching is used to micro-machine fill-tube capsules and target components to allow proper mating for assembly. The fill-tube target is attached to the gas source by a flexible polymer fill line and was tested to confirm gas flow and retention. The effect of the mandrel stalk on the uniformity of GDP coating is investigated.

AbstractEquipment and techniques have been developed for the fabrication of fill tube surrogate targets for OMEGA experiments. The fill tube is attached manually by heating 4000 MW poly-α-methylstyrene in a fixed reservoir, which can be touched onto the capsule surface and pulled into the shape of a fill tube. The joint is uniform and robust with diameters no less than 20 μm. A series of surrogate fill tubes can be achieved by modifying temperature and technique with a diameter reproducibility within 5 μm. After attachment, the capsules are mounted onto a calibrated stage to trim the length of the surrogate to specifications. Characterizing the surrogates involved positioning the polymer stalk to measure the fillet diameter, stalk diameter, and length at orthogonal orientations. Details of the heating and pulling techniques will be discussed as well as a description of the polymer reservoir.

AbstractWe have successfully sputter deposited ≤2 μm thick layers of SiO2on CH mandrels ∼ 2 mm in diameter to act as a permeation barrier for deuterium. Such targets can be used for experiments at Sandia’s Z facility as well as at the National Ignition Facility (NIF). This permeation barrier has been shown to have a half-life (τ1/2) of ∼2–4 weeks for a thickness of ∼ 1.5 μm. The sputter coating conditions have been successfully optimized to produce smooth uniform SiO2coatings with enough integrity to allow routine handling as well as filling to the required pressures (20 atm). The key coating conditions investigated were the agitation mechanism and the coating pressure. We found that an agitation mechanism using gentle rolling produced coatings with a half-life of greater than three weeks, whereas a more vigorous bouncing agitation yielded half-lives of only a few days. Coating pressures of 2, 5, and 10 mTorr were studied and it was found that coatings at 5 mTorr produced coatings free of cracking. Since the sputtering is performed in a background atmosphere of argon, the sputtered SiO2layer was found to contain trace amounts of argon as measured by x-ray fluorescence (XRF) measurements. Our work has yielded a controllable uniform alternative permeation barrier to the traditionally used poly(vinylalcohol) (PVA).

AbstractA mass spectrometer-based system was developed to measure the collected gas permeating through the shell wall of a spherical ICF target. The resultant ion current was used to calculate the material’s permeation half-life. This novel technique is simple, non-destructive, and suitable for measuring targets with short or long half-lives, i.e., very permeable or impermeable target materials, typically in less than 30 min. The technique is suitable for measuring permeation at ambient and elevated temperatures. The results acquired with the mass spectrometer measurements agreed well with those from destructive techniques.

AbstractCooling thin-walled capsules with a high-pressure deuterium fill is a critical phase of operation for providing cryogenic direct-drive targets. During cooling to 20 K, buckling and burst forces develop due to transient thermal gradients, thermal expansion differences in the materials of the capsule and the permeation cell, and changing permeability of the plastic. This article presents the results of both a steady-state and a transient analysis of the pressure differences across the thin-walled capsule during the cooling process. The steady-state contribution to the pressure difference arises from two sources: (1) the different thermal contractions of the materials that comprise the permeation cell and capsule and (2) the room-temperature volume of gas in the line connecting the permeation cell to the isolation valve. The transient analysis considers the pressure differences across the capsule wall that arise from the changing temperature gradients within the gas during the cooling cycle. Both effects have been taken into account to determine an approach that produces fuel-filled, thin-walled cryogenic targets more rapidly. Currently, capsules are slowly cooled at a rate of 0.1 K/min to prevent their destruction. This process requires over 45 h to complete. The results of the present model suggest a faster cooling program that takes into consideration the induced pressure differences, the permeation occurring at higher temperatures, and the strength of the capsule. The time to cool a filled target can be reduced by 25% while maintaining capsule survival.

AbstractThe processing conditions for vapor-depositing polyimide shells were studied to improve the surface finish, tensile properties, and gas permeability for the inertial confinement fusion application. The vapor-deposited (VDP) polyimide possessed distinct properties from solution-cast Kapton, resulting perhaps from its being physically or chemically crosslinked. The VDP polyimide was characterized to be semicrystalline with molecular chains parallel to the shell’s surface. Varying the imidization conditions, i.e., using different atmospheres, heating rates, and heating durations, increased the gas permeability while maintaining the Young’s modulus. Plastically deforming the shells under biaxial stress increased the permeability by up to 1000-fold, which could be reversed when heated to 350°C. Analyses using x-ray diffraction, infrared spectroscopy, and solubility tests indicated that these modifications in properties may have arisen from changes in the crystallinity, crosslinking, and molecular weight. The low-mode (2 to 20) surface roughness was reduced when the shells were slightly inflated; the high-mode roughness (coating-induced bumps) was increased when the substrate was heated to a temperature of 90°C to 140°C.

Freestanding polyimide (PMDA-ODA) thin films (thickness = 1 to 30 µm) and spherical shells (diameter = 950 to 1100 µm, wall thickness = 0.7 to 11.0 µm) have been fabricated by vapour deposition polymerization (VDP). The deposition process was optimized to obtain smooth and uniform deposition composed of equal-molar PMDA and ODA. The tensile properties, including elastic modulus, tensile strength, and elongation at break, were determined by buckling and bursting the shells and using a nanoindentation test on the films. The gas permeability was measured for He, H₂, D₂, O₂ and N₂. The VDP polyimide possessed distinct properties from solution-cast PMDA-ODA polyimide (Kapton^®), including better tensile properties and lower permeability, and was insoluble in concentrated sulfuric acid. The differences in the properties were attributed to the presence of physical or chemical cross-linking in the VDP polyimide. The effects of thermal imidization conditions on the properties were studied, including different heating rates (0.1 to 1.0 °C min^-1), atmospheres (air and N₂), and imidization durations (1 to 6 h). Varying the imidization conditions effectively modified the tensile strength, elongation at break and permeability but had little effect on the elastic modulus. Infrared spectroscopy and solubility tests indicated that the changes in the properties resulted from changes in the molecular weight or degree of cross-linking.

AbstractA thermal model of an OMEGA-scale cryogenic inertial confinement fusion (ICF) target within the layering sphere of the OMEGA Cryogenic Target Positioner was created. A computational fluid dynamics code was used to calculate the temperature profile in the target. The sensitivity of the temperature profiles was modeled for variations in (1) target alignment in the layering sphere, (2) target specifications, and (3) temperature uniformity on the layering sphere’s inner surface. DT-ice non-uniformity in the target was determined by offsetting the center of the ice’s void to a position that minimized the temperature variance of the ice surface. The numerical solutions were compared to analytical solutions when available.

AbstractSpherical polyimide (PMDA-ODA) shells with high aspect ratio (OD = 900 to 1020 μm and wall thickness = 0.7 to 8.0 μm) were successfully fabricated by the vapor deposition method. These shells were characterized in terms of gas permeability, Young’s modulus, tensile strength, and ultimate elongation. The measured properties of the shells agreed with those of commercial films (Kapton®). Post-coating treatments of the shells, including thermal imidization in air and biaxial expansion, effectively increased gas permeability. Air-curing resulted in shells that were more brittle and twice as permeable as those cured in nitrogen. Since no difference in chemical composition was found between the N2- and air-cured shells, the effect of air-curing may be attributed to morphological or microstructural changes due to oxygen or water vapor in air. Shells that were expanded plastically showed over 300-fold increase in gas permeability, while retaining half of the original Young’s modulus. The effect of expansion on permeability may have arisen from localized plastic deformation. Both treatments will be useful for preparing ICF targets that allow a rapid fill with DT fuel for cryogenic experiments.

Hollow polyimide shells, for use as ICF targets, were fabricated by co-depositing monomer precursors from the vapor phase onto bounced spherical mandrels. The process involved two stages: first, the deposited monomers (pyromellitic dianhydride and 4,4′-oxydianiline) reacted on the mandrel surface to form polyamic acid; second, the mandrel was heated to 300°C to imidize the polyamic acid and to decompose the mandrel. During this latter process the decomposed mandrel diffused through the thermally stable coating, leaving a polyimide shell. Depositions were performed under low (∼10−3Torr) and high (∼10−6Torr) vacuum. Also, flat witness films of polyimide deposited on Si wafers and NaCl allowed the mechanical properties and chemical composition of the film during the heating cycle to be measured. Polyimide shells with diameters ranging from 700 to 950 μm and wall thicknesses ranging from 2 to 13 μm were produced. The shell's sphericity was greater than 99%. Burst and buckle pressure tests on these shells yielded the estimated mechanical strength properties. The elastic modulus and tensile strength were ∼15 GPa and ∼300 MPa, respectively. The permeability of D2through polyamic acid at 25°C was 7.4 × 10−17mol·m/m2·Pa·s and increased to 6.4 × 10−16mol·m/m2·Pa·s at 25°C upon curing the shell to 150°C. The permeability of D2at 25°C through vapor-deposited polyimide flat films was measured to be 240 times greater than through the as-deposited polyamic acid, and about 7 times greater than through commer ially available solution-cast Kapton.

AbstractICF experiments routinely make use of capsules filled with precise quantities of gaseous hydrogen and helium isotopes. These two gases in particular readily permeate out of capsules at rates dependent upon variables including shell wall thickness, composition and integrity. Therefore it is important that the fill half-life of these capsules be precisely known so that the exact fill pressure at shot time can be deduced, enabling valid experimental results.This presentation will describe some of our efforts to determine ICF capsule gas fill half-lives. We will compare fill half-life data obtained using weighing, interferometry and mass spectrometry techniques. In addition, we will describe our use of glass shell standards to compare the aforementioned techniques.