Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session PI1: Indirect Drive |
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Chair: David Montgomery, Los Alamos National Laboratory Room: Acadia |
Wednesday, October 29, 2014 2:00PM - 2:30PM |
PI1.00001: Higher Velocity High-Foot Implosions on the National Ignition Facility Laser Invited Speaker: Debra Callahan After the end of the National Ignition Campaign on the National Ignition Facility (NIF) laser, we began a campaign to test capsule performance using a modified laser pulse-shape that delivers higher power early in the pulse (``high foot'') [1,2,3,4]. This pulse-shape trades one-dimensional performance (peak compression) for increased hydrodynamic stability. The focus of the experiments this year have been to improve performance by increasing the implosion velocity using higher laser power/energy, depleted uranium hohlraums, and thinner capsules. While the mix of ablator material into the hotspot has been low for all of these implosions, the challenge has been to keep the implosion shape under control. As the peak laser power is increased, the plasma density in the hohlraum is increased -- making it more and more challenging for the inner cone beams to reach the midplane of the hohlraum and resulting in an oblate implosion. Depleted uranium hohlraums have higher albedo than Au hohlraums, which leads to additional drive and improved implosion shape. Thinner ablators increase the velocity by reducing the amount of payload; thinner ablators also put less mass into the hohlraum which results in improved inner beam propagation. These techniques have allowed us to push the capsule to higher and higher velocity. In parallel with this effort, we are exploring other hohlraums such as the rugby shaped hohlraum to allow us to push these implosions further. This talk will summarize the progress of the high foot campaign in terms of both capsule and hohlraum performance. \\[4pt] [1] H.-S. Park, et al, PRL, 112, 055001 (2014)\\[0pt] [2] T. R. Dittrich, et al, PRL 112, 055002 (2014)\\[0pt] [3] O. A. Hurricane, et al, Nature 506, 343 (2014)\\[0pt] [4] O. A. Hurricane et al., Physics of Plasmas, 21, 056314 (2014) [Preview Abstract] |
Wednesday, October 29, 2014 2:30PM - 3:00PM |
PI1.00002: Three-dimensional simulations of NIF implosions: insight into experimental observations Invited Speaker: Brian Spears We have developed new three-dimensional (3D) radiation hydrodynamics simulation capabilities for NIF implosions to help explain trends in experimental observations. Simulation advances include full Monte Carlo particle transport for nuclear burn and diagnostics processes, resolution of lower-energy DD neutrons, and updated mesh management to allow large, low-mode spherical harmonic perturbations. We have also further advanced our 3D post-processing to generate simulated diagnostics, including time-integrated neutron images, detailed neutron spectra from instrument lines of sight, low-resolution neutron spectra covering the full sphere, and time-resolved x-ray imaging with enhanced spectral resolution. The advanced simulations and the resultant simulated diagnostics reproduce many surprising aspects of the NIF experimental trends. Puzzles that may be explained include longer burn durations, large and strongly direction-dependent ion temperatures as inferred from neutron spectra, large neutron-weighted bulk velocities, and large differences between DD- and DT-inferred temperatures. The improved simulations have allowed the development of controlled NIF experiments designed to test our capabilities to accurately model the impact of 3D shape perturbations on cryogenic layered implosions and surrogate gas-filled symcaps. LLNL-ABS-654616. [Preview Abstract] |
Wednesday, October 29, 2014 3:00PM - 3:30PM |
PI1.00003: Validating Hydrodynamic Growth in National Ignition Facility Implosions Invited Speaker: J. Luc Peterson The hydrodynamic growth of capsule imperfections can threaten the success of inertial confinement fusion implosions. Therefore, it is important to design implosions that are robust to hydrodynamic instabilities. However, the numerical simulation of interacting Rayleigh-Taylor and Richtmyer-Meshkov growth in these implosions is sensitive to modeling uncertainties such as radiation drive and material equations of state, the effects of which are especially apparent at high mode number (small perturbation wavelength) and high convergence ratio (small capsule radius). A series of validation experiments were conducted at the National Ignition Facility to test the ability to model hydrodynamic growth in spherically converging ignition-relevant implosions. These experiments on the Hydro-Growth Radiography platform [1] constituted direct measurements of the growth of pre-imposed imperfections up to Legendre mode 160 and a convergence ratio of greater than four using two different laser drives: a ``low-foot'' drive used during the National Ignition Campaign [2] and a larger adiabat ``high-foot'' drive that is modeled to be relatively more robust to ablation front hydrodynamic growth [3]. We will discuss these experiments and how their results compare to numerical simulations and analytic theories of hydrodynamic growth, as well as their implications for the modeling of future designs. \\[4pt] This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.\\[4pt] [1] K. Raman et al., Phys. Plasmas (accepted 2014); V. Smalyuk et al., Phys. Rev. Lett. (accepted 2014)\\[0pt] [2] M. J. Edwards et al., Phys. Plasmas {\bf 20}, 070501 (2013)\\[0pt] [3] O. Hurricane et al., Nature {\bf 506}, 343 (2014) [Preview Abstract] |
Wednesday, October 29, 2014 3:30PM - 4:00PM |
PI1.00004: Near-vacuum hohlraums for driving fusion implosions with high density carbon ablators Invited Speaker: Laura Berzak Hopkins Achieving ignition requires reaching fast implosion velocities, which highlights the need for a highly efficient hohlraum to drive indirect-drive inertial confinement fusion implosions. Gas-filled hohlraums are typically utilized due to the pulse length (15-20 ns) needed to drive plastic (CH) capsules. With the recent use of 3x denser high-density carbon (HDC) capsules, ignition pulses can be less than 10 ns in duration, providing the opportunity to utilize near-vacuum hohlraums (NVH) to drive ignition-relevant implosions on the National Ignition Facility (NIF) with minimal laser-plasma instabilities which complicate standard gas-filled hohlraums. Initial NVH implosions on the NIF have demonstrated coupling efficiency significantly higher than observed in gas-filled hohlraums -- backscatter losses less than 2{\%} and virtually no suprathermal electron generation. A major design challenge for the NVH is symmetry control. Without tamping gas, the hohlraum wall quickly expands filling the volume with gold plasma. However, results to-date indicate that the inner-cone beams propagate freely to the hohlraum wall for at least 6.5 ns. With minimal predicted cross-beam power transfer, this propagation enables symmetry control via dynamic beam phasing -- time-dependent direct adjustment of the inner- and outer-cone laser pulses. A series of experiments with an HDC ablator and NVH culminated in a 6 ns, 1.2 MJ cryogenic DT layered implosion yielding 1.8 x 10$^{\mathrm{15}}$ neutrons---significantly higher yield than any CH implosion at comparable energy. This implosion reached an ignition-relevant velocity -- 350 km/s -- with no observed ablator mix in the hot spot. Recent experiments have explored two-shock designs in a larger, 6.72 mm hohlraum, and upcoming experiments will incrementally extend the pulse duration toward a 9 ns long, three-shock ignition design. [Preview Abstract] |
Wednesday, October 29, 2014 4:00PM - 4:30PM |
PI1.00005: In-flight Density Profiles and Areal Density Non-uniformities of ICF Implosions Invited Speaker: Riccardo Tommasini Implosion efficiency depends on keeping the in-flight ablator and fuel as close as possible to spherical at all times while maintaining the required implosion velocity and in-flight aspect ratio. Asymmetries and areal density non-uniformities seeded by time-dependent drive variations and target imperfections grow in time as the capsule implodes, with growth rates that are amplified by instabilities. One way to diagnose them is by imaging the self-emission from the implosion core. However this technique, besides only providing direct information of the shape of the hot emission region at final assembly, presents complications due to competition between emission gradients and reabsorption. Time resolved radiographic imaging, being insensitive to this effect, is therefore an important tool for diagnosing the ablator and the shell in inertial confinement fusion (ICF) implosions. Experiments aimed at measuring the density, areal density and areal density asymmetries of the shell in ICF implosions have been performed using two different radiography techniques on the National Ignition Facility. We will report the results from both 1D [1] and 2D [2] geometries using slit and pinhole imaging coupled to area backlighting and as close as 150 ps to peak compression. We will focus in particular on comparisons of the in-flight shell thicknesses and ablation front scale lengths between low- and high-adiabat [3] implosions, and the perturbations on areal density seeded both by time dependent drive asymmetries and by the membranes used to hold the capsule within the hohlraum in indirect drive ICF targets.\\[4pt] [1] Hicks, D. G. et al., Phys Plasmas 19, 122702 (2012)\\[0pt] [2] R. Rygg, et. al., Phys. Rev. Lett. 112 195001 (2014)\\[0pt] [3] O. Hurricane, et al., Nature 1--7 (2014) [Preview Abstract] |
Wednesday, October 29, 2014 4:30PM - 5:00PM |
PI1.00006: Studying shock dynamics and in-flight $\rho $R asymmetries in NIF implosions using proton spectroscopy Invited Speaker: Alex Zylstra Ignition-scale, indirect-drive implosions of CH capsules filled with D$^{3}$He gas have been studied with proton spectroscopy at the NIF. Spectral measurements of D$^{3}$He protons produced at the shock-bang time provide information about the shock dynamics and in-flight characteristics of these implosions. The observed energy downshift of the D$^{3}$He-proton spectra are interpreted with a self-consistent 1-D model to infer $\rho $R, shell R$_{cm}$, and yield at this time. The observed $\rho $R at shock-bang time is substantially higher for implosions where the laser drive is on until near the compression-bang time (``short-coast'') while longer-coasting implosions generate lower $\rho $R at shock-bang time. This is most likely due to a larger temporal difference between the shock- and compression-bang time in the long-coast implosions ($\sim$800ps) than in the short-coast implosions ($\sim$400ps). These differences are determined from the D$^{3}$He proton spectra and in-flight x-ray radiography data, and it is found to contradict radiation-hydrodynamic simulations, which predict a 700 -- 800ps temporal difference independent of coasting time. A large variation in the shock proton yield is also observed in the dataset, which is interpreted with a Guderley shock model and found to correspond to $\sim 2 \times$ variation in incipient hot-spot adiabat caused by shock heating. This variation may affect the compressibility of NIF implosions. Finally, data from multiple proton spectrometers placed at the pole and equator reveal large $\rho $R asymmetries, which are interpreted as mode-2 polar or azimuthal asymmetries. At the shock-bang time (CR $\sim 3-5$), asymmetry amplitudes $\ge $10{\%} are routinely observed. Compared to compression-bang time x-ray self-emission symmetry, no apparent asymmetry-amplitude growth is observed, which is in contradiction to several growth models. This is attributed to a lack of correspondence between shell and hot-spot symmetry at peak compression, as discussed in recent computational studies [R.H.H. Scott et al., Phys. Rev. Lett. 110, 075001 (2013)]. [Preview Abstract] |
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