Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session YO5: Target Fabrication and HEDP |
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Chair: Gordon Chandler, Sandia National Laboratories Room: Rosen Centre Hotel Salon 11/12 |
Friday, November 16, 2007 9:30AM - 9:42AM |
YO5.00001: Forming Cryogenic DT Targets for OMEGA D.R. Harding, D.H. Edgell, L.M. Elasky Experiments at LLE have shown that it is possible to fill thin-walled plastic capsules with a mixture of deuterium--tritium (DT) in 3 days and form a 70{\-}~to 95-\textit{$\mu $}m ice layer with an inner surface roughness better than 1-\textit{$\mu $}m rms in an additional day. Of the 33 DT targets that have been processed, the average ice roughness is 1.4$\pm $0.9 \textit{$\mu $}m with a median value of 1.4 \textit{$\mu $}m; eight of the targets had a roughness below 1.0-\textit{$\mu $}m rms. The smoothest ice layer possessed a roughness of 0.5-\textit{$\mu $}m rms, a value where the roughness of the plastic capsule's surface and the sensitivity of the shadowgraphy technique begin to affect the accuracy of the analysis. Repeated melting and relayering of the ice layer in the same capsule yielded sub{\-}1{\-}\textit{$\mu $}m-rms ice roughness each time. The residual roughness is controlled by the crystal growth process, which requires that an initial single crystal be nucleated within the liquid DT and that its growth rate be finely controlled; if the crystal grows too rapidly, facets and dislocations form within the ice. The effect of these crystalline defects is to distort the shadowgraphic analysis. Other notable observations have been (1) that no $^{3}$He bubbles were observed in the DT-ice layers after the layer had aged for 19 days, and (2) that transparent DT-ice layers were formed in foam targets (0.16- and 0.116-g/cm$^{3}$ foam density). This work was supported by the U.S. DOE under Cooperative Agreement DE-FC52-92SF19460. [Preview Abstract] |
Friday, November 16, 2007 9:42AM - 9:54AM |
YO5.00002: Development of a laser-produced plasma x-ray source for phase-contrast imaging of DT fuel ice layers N. Izumi, E. Dewald, B. Kozioziemski, J.A. Koch Because beryllium capsules for NIF experiments are not transparent to visible light, optical microscopy is not applicable for metrology of deuterium-tritium (DT) ice layers. X-ray absorption radiography cannot be used either because absorption in DT ice is negligible, so to quantify the quality of the DT ice surface, x-ray phase-contrast imaging is used in order to enhance contrast of surface imperfections. Phase contrast imaging of ice layers typically utilizes micro-focus x-ray tube sources, but available x-ray fluxes are limited, and these sources cannot be used to quantify changes in the ice surface quality over the second timescales appropriate for rapidly-cooled layers. We have therefore explored the use of a laser-produced plasma x-ray source in order to determine if it has sufficient brightness to produce high-quality phase-contrast flash radiographs of DT ice layers. We irradiated Ti, Fe, Cu, and Au targets with 5-ns, 300-J, 527-nm laser light at the Janus laser facility, and measured absolute x-ray conversion efficiency and x-ray spot size. We will discuss this data as well as phase-contrast radiographs we obtained of non-cryogenic shells. [Preview Abstract] |
Friday, November 16, 2007 9:54AM - 10:06AM |
YO5.00003: Target Fabrication in Support of Inertial Confinement Fusion and High Energy Density Physics Experiments C.A. Back, A. Nikroo, J.D. Kilkenny, A.V. Hamza, L.J. Atherton Target fabrication is increasingly important in experiments for inertial confinement fusion and high energy density physics. As the facilities are nearing completion and programs become more mature, refinements and new target designs remain a path to explore new physics through development of new materials and techniques. To meet the needs of the laboratory and academic communities, the target fabrication groups are working together across the DOE and NNSA complex to make best use of facilities and capabilities at the different sites. The challenge of target fabrication is to place new materials with sub-micron to micron tolerances into mm-sized packages. Targets are often needed in limited quantities, and even for a point design, flexible fabrication support is needed to meet evolving needs, making it difficult to take advantage of economies of scale. Here, we highlight the target ordering process we have implemented to specify, track, and use resources as efficiently as possible. [Preview Abstract] |
Friday, November 16, 2007 10:06AM - 10:18AM |
YO5.00004: Fabrication and Metrology of Ignition Design Graded Copper Doped Beryllium Capsules With Fill Tubes A. Nikroo, M.S. Conyers, S.A. Eddinger, A.C. Forsman, H. Huang, E.H. Lundgren, K.A. Moreno, H. Xu, K.P. Youngblood, Z.A. Zimmerer, S.D. Bhandarkar, C.S. Alford, S.A. Letts, J.L. Reynolds Graded doped beryllium is currently the baseline ablator in designs for achieving ignition in the National Ignition Facility (NIF). In this paper, we report on the progress towards fabricating and characterizing such beryllium capsules for NIF ignition designs. We have eliminated the unanticipated gas leakage observed in graded copper doped beryllium capsules made by sputter coating by ion-assisted deposition. Polishing and mandrel selection render shells that meet the surface roughness. Precision radiography measures azimuthal x-ray optical depth of the shell to 10$^{-4}$. Laser drilling produces 5 $\mu $m holes in full thickness shells. Progress towards drilling a counter bore and 10 $\mu $m fill tube attachment to ignition design will be discussed. Improvements in metrology of Cu dopant and contaminant profile using quantitative contact and electron probe techniques will be also presented. [Preview Abstract] |
Friday, November 16, 2007 10:18AM - 10:30AM |
YO5.00005: Fabrication of Embedded Spheres in Resorcinol Formaldehyde Aerogel for Astrophysical Jet Experiments on OMEGA R.R. Paguio, J.F. Hund, B.E. Blue, D.G. Schroen, K.M. Saito, C.A. Frederick, R.J. Strauser, K. Quan Embedded sapphire spheres in a cylindrical resorcinol formaldehyde aerogel targets are required for astrophysical jet studies on OMEGA. Previous work done on fabricating such a target used a 100 $\mu \mbox{m}$ thick stalk in order to place the ball in the required position. Recent experiments required the stalk to be 1 $\mu \mbox{m}$ or less, and changed the aerogel density from 100~mg/cc to 300 mg/cc. Targets were successfully fabricated without a supporting stalk by modifying parameters such as gelation time of the aerogel precursor solution, and use of a vacuum holder for the sphere placement. Fabrication of this 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 aerogel was done by radiography. These measurements showed that the ball was accurately placed and the aerogel matrix was not significantly altered. [Preview Abstract] |
Friday, November 16, 2007 10:30AM - 10:42AM |
YO5.00006: Fabrication of Astrophysically Relevant Targets for Use on Inertial Confinement Fusion Machines B.E. Blue, C.A. Back, J.F. Hund, R. Paguio, D.G. Schroen, J.M. Foster, P.A. Rosen, R.J.R. Williams, B.H. Wilde, R.F. Coker, R. Wallace, J.F. Hansen, R.M. Stamm, J. Palmer, R. Carver, P. Hartigan A series of experiments has been conducted on the Omega laser at the University of Rochester that scale to astrophysical jets. We have fielded experiments to study the hydrodynamic evolutions of high-Mach-number jets, jets deflecting from a high-density sphere that simulate astrophysical jets interacting with stellar clouds at different impact parameters, jets evolving into foams of varying cell size to understand the effect of medium inhomogeneity, and shocks impacting a dense sphere to simulate shocks interacting with gas clouds. This talk will present the target fabrication challenges related to these experiments. Such challenges include foam production, precisely embedding objects in foams, assembling the multiple components with tight tolerances, and the extensive metrology and characterization that is needed to accurately model, and derive results from, these experiments. [Preview Abstract] |
Friday, November 16, 2007 10:42AM - 10:54AM |
YO5.00007: Fabrication of Planar Foam Targets for Rayleigh-Taylor Instability Experiments C.A. Frederick, R.R. Paguio, J.F. Hund Cryogenic direct-drive inertial confinement fusion (ICF) experiments at the Omega Laser Facility require 180 mg/cc resorcinol formaldehyde (RF) foam shells overcoated with a full density layer of CH. Surface perturbations on the overcoated shells can amplify Rayleigh-Taylor instabilities during target implosion. To study Rayleigh-Taylor instability, planar foam targets with a full density CH coating were fabricated. In an effort to also better understand the different modes of instability single sine waves with varying wavelengths and amplitudes were laser machined into the full density layer of CH. Fabrication of the planar foam pieces, application of the full density CH layer, and laser patterning will be discussed. Characterization of the laser-machined patterns was done using interferometric and radiographic techniques. [Preview Abstract] |
Friday, November 16, 2007 10:54AM - 11:06AM |
YO5.00008: Density Uniform and Surface Characterization of Tantalum Oxide Aerogel for Radiation Transport Experiments J.F. Hund, C.A. Frederick, A.P. Tipton, E.M. Giraldez, J.L. Kaae, C.A. Back, M.L. Hoppe Jr Tantalum aerogel was fabricated and machined at General Atomics for radiation transport experiments at the OMEGA laser facility. These targets are machined into small disks from 250 or 500 mg/cc tantalum oxide aerogel. During machining, differences in density uniformity and machining characteristics were observed in some of the aerogels. Ultimately, rough surfaces or intrinsic density variations in the sample can lead to areal density non-uniformities that can affect the experiment. Using contact radiography, optical profilometry, and scanning electron microscopy the surface roughness and areal density uniformity were quantified. This information was used to improve the synthesis procedure of the bulk aerogel material to fabricate more uniform aerogel targets and provide input for comparison shot data of experiment vs modeling data. [Preview Abstract] |
Friday, November 16, 2007 11:06AM - 11:18AM |
YO5.00009: Fundamental Limits on the Maximum Aspect Ratios of Laser Drilled Holes A.C. Forsman, E.H. Lundgren, A.M. Komashko The drilling of $<$6 $\mu $m diameter holes in 170 $\mu $m deep shells has been demonstrated using a nanosecond laser system that produces a formatted pulse output where each laser shot consists of a pair of timed nanosecond laser pulses. This work was done to enable gas fills in beryllium capsules for inertial confinement fusion experiments. This is an involved goal in laser process development. The interplay of material characteristics, hydrodynamic flows, and laser-matter interactions have been studied. The drilling process will be described, as well as possible limitations on the maximum ratio of hole depth to hole width that are imposed by the laser matter interactions, the material properties and the laser produced plasmas themselves. [Preview Abstract] |
Friday, November 16, 2007 11:18AM - 11:30AM |
YO5.00010: LASNEX Radiation Wave Simulations and Implications for Temperature Measurements Heidi Tierney, Robert Peterson, Paul Keiter, Thomas Tierney, Glenn Magelssen LANL is developing a diagnostic platform that will allow temperature measurements of a radiation front as it propagates through a medium. These diagnostic development experiments, called NIF Platform Five, are being performed at University of Rochester's 60-beam OMEGA laser. The OMEGA NIF-5 experiments use a 3/4-scale hohlraum mounted on the P6-P7 axis. Fifteen beams are used to drive the hohlraum to radiation temperatures near 220-240 eV in a 1-ns pulse. A radiation wave flows through a 60-mg/cc CHCl foam, which is mounted in a cylinder on the opposite side of the hohlraum from the 0.8-mm diameter LEH. A backlighter is used to perform measurements of absorption spectroscopy for electron temperature determination, while self emission is the primary process that allows measurement of radiation front position. We present 2-D R-Z LASNEX simulations of data taken at OMEGA using a soft x-ray framing camera in January 2007. The simulated laser drive, target shape, and material composition replicate experimental conditions for individual shots as closely as possible. Post processing tools for radiation transport and filtering have been used for final comparison with the soft x-ray camera measurements. Data comparison to simulation and implications for temperature measurements will be discussed. [Preview Abstract] |
Friday, November 16, 2007 11:30AM - 11:42AM |
YO5.00011: Measurements of X-ray energy flow through evolving density gradients to validate the modeling of stellar atmospheres P. Graham, J. Foster, A. Moore, M. Taylor, S. MacLaren, P. Young, G. Glendinning, A. Reighard, C. Sorce, C. Back, J. Hund, B. Blue Density perturbations, such as N-waves, in stellar atmospheres are coupled to the X-ray radiation field and so their evolution is challenging to simulate [1]. To assess current modeling capabilities an analogous problem was generated on the LLE OMEGA laser using a hohlraum to drive X-rays through tantalum aerogel with an initial seed perturbation. X-rays diffuse preferentially through the lower density material and the flow changes over time as the heated mass evolves. The energy flow was diagnosed using two methods, direct flux and hohlraum calorimetry, which are compared to assess the best technique. In both cases multiple flux diagnostics on different lines of sight were used, including photodiode and photoconductive detectors, to crosscheck results. In addition, 2D framing images of X-ray emission were taken to correlate with the flux measurements. The suite of data will be presented and compared against modeling. [1] Mihalas {\&} Mihalas, `Foundations of Radiation Hydrodynamics', Dover (1999). [Preview Abstract] |
Friday, November 16, 2007 11:42AM - 11:54AM |
YO5.00012: On-axis Hohlraum Radiography associated with N-Waves in Stellar Atmospheres. A.S. Moore, J. Foster, P. Graham, M. Taylor, S. MacLaren, P. Young, G. Glendinning, A. Reighard, C. Sorce, C. Back, J. Hund, B. Blue The propagation of weak shocks in a stellar atmosphere, in conjunction with the high x-ray flux cannot be well-described using weak-shock theory. Experiments performed at the LLE OMEGA laser attempt to study shock dynamics similar radiation conditions. Point-projection radiography was performed along the axis of a 160eV hohlraum, illuminating the structures formed by the ablation of a 0.2mm annular slot in a solid Ta disc. Diagnosed whilst laser-driven, the platform also enables quantitative measurements of x-ray flow through high-Z foam slot. Backlit images of the radiatively-driven slot show complex `bubble-like' features at the intersection of ablation fronts. Despite the 3D aspects of the experiment, 2D simulations, using the radiation-hydrodynamics code are an excellent qualitative match to the data, demonstrating that structures result from a high pressure spike that forms from the colliding ablation fronts driving a blast wave-like expansion into the dense stagnation region. [Preview Abstract] |
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