Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session TO5: Stagnation I |
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Chair: Maria Gatu Johnson, Massachusetts Institute of Technology Room: 230 B |
Thursday, November 3, 2016 9:30AM - 9:42AM |
TO5.00001: Ignition and Thermonuclear Burn on the National Ignition Facility with Imposed Magnetic Fields L. John Perkins, B.G. Logan, M.A. Rhodes, G.B. Zimmerman, D.D. Ho, D.T. Blackfield, S.A. Hawkins We are studying the impact of highly compressed magnetic fields on enhancing the prospects for ignition and burn on the National Ignition Facility (NIF). Both magnetized room-temperature DT gas targets and cryo-ignition capsules are under study. Applied seed fields of 20-70T that compress to greater than 10000T (100MG) under implosion can reduce hotspot conditions required for ignition and propagating burn through range reduction and magnetic mirror trapping of fusion alpha particles, suppression of electron heat conduction and potential stabilization of hydrodynamic instabilities. The applied field may also reduce hohlraum laser-plasma instabilities and suppress the transport of hot electron preheat to the capsule. These combined B-field attributes may permit recovery of ignition, or at least significant alpha particle heating, in capsules that are otherwise submarginal through adverse hydrodynamic or hohlraum-drive conditions. Simulations indicate that optimum initial fields of 50T may produce multi-MJ-yields when applied to our present best experimental capsules. Proof-of-principle experiments for magnetized ignition capsules and hohlraum physics on NIF are now being designed. [Preview Abstract] |
Thursday, November 3, 2016 9:42AM - 9:54AM |
TO5.00002: Turbulent mix or ion diffusion? Hypothesis testing in ICF capsule implosions N M Hoffman, G B Zimmerman, S A Vander Wiel, H W Herrmann, Y H Kim Turbulent mixing at a contact surface combines materials that are initially separated across the contact. While the mixing layer may contain the initially separate materials (each assumed to be composed of a number of distinct ion species) in a range of concentrations, from zero to 100{\%}, the concentration of individual ion species within each material, relative to one another, is not altered by turbulent mixing alone. Ion diffusion likewise causes mixing at a contact, but does alter the relative concentration of ion species within each material, since the relative diffusivity of ions, in a fixed background plasma, varies as \textasciitilde $A^{\mathrm{1/2}}$/$Z^{\mathrm{2}}$. Recent hydrodynamically equivalent capsule implosions allow a test of the influence of these processes on observed capsule behavior. We use numerical simulations and hypothesis-testing methods to show quantitatively that turbulent mixing with ion diffusion is a better explanation of observed behavior than turbulent mixing alone (subject to the assumptions inherent in the computational models of these processes.) [Preview Abstract] |
Thursday, November 3, 2016 9:54AM - 10:06AM |
TO5.00003: High-Performance Cryogenic Designs for OMEGA and the National Ignition Facility V.N. Goncharov, T.J.B. Collins, J.A. Marozas, S.P. Regan, R. Betti, T.R. Boehly, E.M. Campbell, D.H. Froula, I.V. Igumenshchev, R.L. McCrory, J.F. Myatt, P.B. Radha, T.C. Sangster, A. Shvydky The main advantage of laser symmetric direct drive (SDD) is a significantly higher coupled drive laser energy to the hot-spot internal energy at stagnation compared to that of laser indirect drive. Because of coupling losses resulting from cross-beam energy transfer (CBET), however, reaching ignition conditions on the NIF with SDD requires designs with excessively large in-flight aspect ratios $\left( {\sim 30} \right).$ Results of cryogenic implosions performed on OMEGA show that such designs are unstable to short-scale nonuniformity growth during shell implosion. Several CBET reduction strategies have been proposed in the past.\footnote{I. V. Igumenshchev\textit{ et al.}, Phys. Plasmas \textbf{19}, 056314 (2012); D. H. Froula\textit{ et al.}, Phys. Plasmas \textbf{20}, 082704 (2013).} This talk will discuss high-performing designs using several CBET-mitigation techniques, including using drive laser beams smaller than the target size and wavelength detuning. Designs that are predicted to reach alpha burning regimes as well as a gain of $\sim {\kern 1pt}10$ to 40 at the NIF-scale will be presented. Hydrodynamically scaled OMEGA designs with similar CBET-reduction techniques will also be discussed. This material is based upon work supported by the Department Of Energy National Nuclear Security Administration under Award Number DE{\-}NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 10:06AM - 10:18AM |
TO5.00004: Hydrodynamic Mixing of Ablator Material into the Compressed Fuel and Hot Spot of Direct-Drive DT Cryogenic Implosions S.P. Regan, V.N. Goncharov, R. Epstein, R. Betti, M.J. Bonino, D. Cao, T.J.B. Collins, E.M. Campbell, C.J. Forrest, V.Yu. Glebov, D.R. Harding, J.A. Marozas, F.J. Marshall, P.W. McKenty, T.C. Sangster, C. Stoeckl, R.W. Luo, M.E. Schoff, M. Farrell Hydrodynamic mixing of ablator material into the compressed fuel and hot spot\footnote{S. P. Regan\textit{ et al.}, Phys. Rev. Lett. \textbf{111}, 045001 (2013).\par } of direct-drive DT cryogenic implosions is diagnosed using time-integrated, spatially resolved x{\-}ray spectroscopy. The laser drive ablates most of the 8-$\mu $m-thick CH ablator, which is doped with trace amounts of Ge $\left( {\sim 0.5\mbox{\thinspace at.\thinspace \% }} \right)$ and surrounds the cryogenic DT layer. A small fraction of the ablator material is mixed into the compressed shell and the hot spot by the ablation-front Rayleigh--Taylor hydrodynamic instability seeded by laser imprint, the target mounting stalk, and surface debris. The amount of mix mass inferred from spectroscopic analysis of the Ge K-shell emission will be presented. This material is based upon work supported by the Department Of Energy National Nuclear Security Administration under Award Number DE{\-}NA0001944. Part of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE{\-}AC52{\-}07NA27344. [Preview Abstract] |
Thursday, November 3, 2016 10:18AM - 10:30AM |
TO5.00005: Simulation and Analysis of Time-Resolved Narrowband Radiographs of Cryogenic Implosions on OMEGA R. Epstein, C. Stoeckl, V.N. Goncharov, P.W. McKenty, S.P. Regan Spherical polymer shells containing cryogenic DT ice layers have been imploded on the OMEGA Laser System and radiographed with Al backlighter targets $\left( {h\nu =1.865\mbox{\thinspace keV}} \right)$ driven with 20-ps IR pulses from the OMEGA EP Laser System. X-ray radiographs have been simulated using \textit{DRACO} and \textit{Spect3D}. The shadows of the converging DT ice and polymer shell edges at times before and after stagnation are visible while the self-emission is minimized using a time-resolved (40-ps) narrowband crystal imaging system. The self-emission from the diverging shock wave following stagnation is also visible. The simulated radiographs will be compared to the measured ones to investigate the feasibility of diagnosing the low-mode asymmetry in the compressed DT shell around stagnation. This material is based upon work supported by the U.S. Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 10:30AM - 10:42AM |
TO5.00006: Measurements of the Effect of Adiabat on Shell Decompression in Direct-Drive Implosions on OMEGA D.T. Michel, S.X. Hu, P.B. Radha, A.K. Davis, R.S. Craxton, V.Yu. Glebov, V.N. Goncharov, I.V. Igumenshchev, C. Stoeckl, D.H. Froula Measurements of the effect of adiabat $\left( \alpha \right)$ on the shell thickness were performed in direct-drive implosions. The maximum in-flight shell thickness was obtained using a novel technique where the outer and inner surfaces of the shell were simultaneously measured using self-emission images of the imploding target. When reducing the shell's adiabat from $\alpha =6$ to $\alpha =4.5,$ the shell thickness was measured to decrease from $75\mu \mbox{m}$ to $60\mu \mbox{m,}$ but when decreasing the adiabat further $\left( {\alpha =1.8} \right),$ the shell thickness was measured to increase to $75\mu \mbox{m.}$ The measured shell thickness, shell trajectories, neutron bang time, and neutron yield were reproduced by two-dimensional simulations that include laser imprint, nonlocal thermal transport, cross-beam energy transfer, and first-principles equation-of-state models. These results show that the decompression of the shell measured for low-adiabat implosions was a result of laser imprint. Additional information on the evolution of the density profile was obtained using x-ray radiography. The backlighter was created with six of the 60 OMEGA laser beams, with the pointings and energies of other beams adjusted to maintain a uniform implosion. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 10:42AM - 10:54AM |
TO5.00007: Mitigating the impact of hohlraum asymmetries in National Ignition Facility implosions using capsule shims Daniel Clark, Christopher Weber, Vladimir Smalyuk, Harry Robey, Andrea Kritcher, Jose Milovich, Jay Salmonson Current indirect drive implosion experiments on the National Ignition Facility (NIF) [E. I. Moses, R. N. Boyd, B. A. Remington, C. J. Keane, and R. Al-Ayat, Phys. Plasmas \quad \textbf{16}, 041006 (2009)] are believed to be strongly impacted by long wavelength perturbations driven by asymmetries in the hohlraum x-ray flux. To address this perturbation source, active efforts are underway to develop modified hohlraum designs with reduced asymmetry imprint. An alternative strategy, however, is to modify the capsule design to be more resilient to a given amount of hohlraum asymmetry. In particular, the capsule may be deliberately misshaped, or ``shimmed,'' so as to counteract the expected asymmetries from the hohlraum. Here, the efficacy of capsule shimming to correct the asymmetries in two recent NIF implosion experiments is assessed using two-dimensional radiation hydrodynamics simulations. Despite the highly time-dependent character of the asymmetries and the high convergence ratios of these implosions, simulations suggest that shims could be highly effective at counteracting current asymmetries and result in factors of a few enhancements in neutron yields. For higher compression designs, the yield improvement could be even greater. [Preview Abstract] |
Thursday, November 3, 2016 10:54AM - 11:06AM |
TO5.00008: Impact of flows on ion temperatures inferred from neutron spectra in asymmetrically driven OMEGA DT implosions M. Gatu Johnson, J.A. Frenje, F.H. Seguin, R.D. Petrasso, B. Aappelbe, J. Chittenden, C. Walsh, J.P. Knauer, V.YU. Glebov, C. Forrest, F. Marshall, T. Michel, C. Stoeckl, T.C. Sangster, A. Zylstra Ion temperatures ($T_{\mathrm{ion}})$ in Inertial Confinement Fusion (ICF) experiments have traditionally been inferred from the broadening of primary neutron spectra. Directional motion (flow) of the fuel at burn, expected to arise due to asymmetries imposed by engineering features (such as stalks, fill tubes, tents, or capsule imperfections) or drive non-uniformity, also impacts broadening and may lead to artificially inflated ``$T_{\mathrm{ion}}$'' values. Flow due to low-mode asymmetries is expected to give rise to line-of-sight variations in measured $T_{\mathrm{ion}}$, as observed in OMEGA cryogenic DT implosions but not in similar experiments at the NIF. In this presentation we report on an OMEGA experiment with intentionally asymmetric drive, designed to test the ability to accurately predict and measure line-of-sight differences in apparent $T_{\mathrm{ion}}$ due to low-mode asymmetry-seeded flows. The results provide insight into the complexity of hot-spot dynamics, which is a problem that must be mastered to achieve ICF ignition. [Preview Abstract] |
Thursday, November 3, 2016 11:06AM - 11:18AM |
TO5.00009: Large Survey of Neutron Spectrum Moments Due to ICF Drive Asymmetry J.E. Field, D. Munro, B. Spears, J.L. Peterson, S. Brandon, J.A. Gaffney, J. Hammer, S. Langer, R.C. Nora, P. Springer We have recently completed the largest HYDRA simulation survey to date (~$\approx 60,000$ runs) of drive asymmetry on the new Trinity computer at LANL. The 2D simulations covered a large space of credible perturbations to the drive of ICF implosions on the NIF. Cumulants of the produced birth energy spectrum for DD and DT reaction neutrons were tallied using new methods.\footnote{see D.H.~Munro, \emph{Nucl. Fusion}, 56 (2016) 036001.} Comparison of the experimental spectra with our map of predicted spectra from simulation should provide a wealth of information about the burning plasma region. We report on our results, highlighting areas of agreement (and disagreement) with experimental spectra. We also identify features in the predicted spectra that might be amenable to measurement with improved diagnostics. Prepared by LLNL under Contract DE-AC52-07NA27344.\footnote{IM release \#: LLNL-PROC-697321} [Preview Abstract] |
Thursday, November 3, 2016 11:18AM - 11:30AM |
TO5.00010: Measurement of fluid motion using antipodal nToF detectors. Gary Grim, Richard Bionta, Jac Caggiano, Mark Eckart, Ed Hartouni, Robert Hatarik, Joe Kilkenny, Alastair Moore, Daniel Sayre, Charles Yeamans The mid-2016 implementation of a neutron time-of-flight (nToF) detector in the northern hemisphere of the NIF experimental areas has provided an approximately antipodal detector pair configuration. In addition to enabling the first measurements of neutron spectra in the northern hemisphere of the NIF Target Bay, this configuration enables the most sensitive measurement of north/south fluid motion during neutron production in inertial confinement fusion implosions. We present the status and initial results of measurements using the NIF antipodal nToF system. [Preview Abstract] |
Thursday, November 3, 2016 11:30AM - 11:42AM |
TO5.00011: Evaluation of the Effects of Long-Wavelength Perturbations in OMEGA 80-Gbar Cryogenic Implosions P.W. McKenty, D. Cao, T.J.B. Collins, A. Shvydky, K.S. Anderson The Laboratory for Laser Energetics, as part of the National Laser Direct Drive Program, has identified the goal of producing 100-Gbar neutron-averaged, hot-spot pressures ($P$*) by the year 2020. An intermediate goal of 80 Gbar is currently being pursued. This work first analyzes the behavior of $P$* as a function of the target convergence ratio. From this a critical converge ratio can be defined at which point the implosion achieves the $P$* $=$ 80-Gbar goal. Further capsule convergence then maps out a target region in design space that details the acceptable degradation from 1-D performance an implosion could suffer while still achieving the 80-Gbar goal. Two-dimensional simulation results will be presented, indicating the maximum-allowed levels for long-wavelength perturbations (offset, power imbalance, and inner-surface ice roughness) while still completing this goal. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE{\-}NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 11:42AM - 11:54AM |
TO5.00012: Investigation of Acquired Fuel Motion Caused by Ice Roughness in OMEGA Cryogenic Experiments D. Cao, P.W. McKenty, J.P. Knauer It is expected that DT ice/gas interfaces in cryogenic targets will have a certain level of ice roughness; however, less is known about the possible influence of this roughness on net fuel motion during a target implosion. Measureable nonzero net fuel velocity is typically associated with low-$\ell $ mode asymmetries. Since ice roughness is mainly characterized by low $\ell $ modes, this work examines the effect of roughness on fuel motion in OMEGA cryogenic experiments. The measurements of fuel motion are taken using neutron time-of-flight (nTOF) diagnostics, which operate on the principle that emitted neutrons have an additional velocity component caused by the fluid motion from which they are borne. This gives rise to an energy shift of the neutron energy spectra. NTOF measurements will be shown illustrating the overall fuel motion that is systematically seen in OMEGA cryogenic implosions but not seen in warm target implosions. Results from 2-D \textit{DRACO} simulations, which include low $\ell $-mode ice roughness, will be presented and the predicted acquired fuel motion will be compared to experimental data. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, November 3, 2016 11:54AM - 12:06PM |
TO5.00013: Dynamically stable implosions in a large simulation dataset J. Luc Peterson, John Field, Kelli Humbird, Scott Brandon, Steve Langer, Ryan Nora, Brian Spears Asymmetric implosion drive can severely impact the performance of inertial confinement fusion capsules. In particular the time-varying radiation environment produced in near-vacuum hohlraum experiments at the National Ignition Facility is thought to limit the conversion efficiency of shell kinetic energy into hotspot internal energy. To investigate the role of dynamic asymmetries in implosion behavior we have created a large database of 2D capsule implosions of varying drive amplitude, drive asymmetry and capsule gas fill that spans 13 dimensions and consists of over 60,000 individual simulations. A novel in-transit analysis scheme allowed for the real-time processing of petabytes of raw data into hundreds of terabytes of physical metrics and synthetic images, and supervised learning algorithms identified regions of parameter space that robustly produce high yield. We will discuss the first results from this dataset and explore the dynamics of implosions that produce significant yield under asymmetric drives. [Preview Abstract] |
Thursday, November 3, 2016 12:06PM - 12:18PM |
TO5.00014: Surrogate models for identifying robust, high yield regions of parameter space for ICF implosion simulations Kelli Humbird, J. Luc Peterson, Scott Brandon, John Field, Ryan Nora, Brian Spears Next-generation supercomputer architecture and in-transit data analysis have been used to create a large collection of 2-D ICF capsule implosion simulations. The database includes metrics for approximately 60,000 implosions, with x-ray images and detailed physics parameters available for over 20,000 simulations. To map and explore this large database, surrogate models for numerous quantities of interest are built using supervised machine learning algorithms. Response surfaces constructed using the predictive capabilities of the surrogates allow for continuous exploration of parameter space without requiring additional simulations. High performing regions of the input space are identified to guide the design of future experiments. In particular, a model for the yield built using a random forest regression algorithm has a cross validation score of 94.3\% and is consistently conservative for high yield predictions. The model is used to search for robust volumes of parameter space where high yields are expected, even given variations in other input parameters. Surrogates for additional quantities of interest relevant to ignition are used to further characterize the high yield regions. [Preview Abstract] |
Thursday, November 3, 2016 12:18PM - 12:30PM |
TO5.00015: Three-Dimensional Study of Yield Degradation for Direct-Drive Inertial Confinement Fusion K.M. Woo, R. Betti, R. Yan, H. Aluie, A. Bose, D.X. Zhao, V. Gopalaswamy The mechanism of yield degradation in the deceleration phase for direct-drive inertial confinement fusion was studied using a recently developed three-dimensional radiation--hydrodynamics code \textit{DEC3D}. Under the approximation of adiabatic hot spot, an expression that measures the degradation of neutron rate was obtained in terms of the ratio of perturbed to the clean hot-spot volume. The characteristics of perturbed hot-spot volume is identified as a key parameter to understand the departure from spherical symmetry. The role of 3-D effects on compressibility, which affects the hot-spot volume, was examined including the 3-D vorticity dynamics in the spherical converging geometry and the jet flow in P-1 perturbations. In particular, the hot spot was found to be less compressible in the nonlinear phase of the Rayleigh--Taylor instability, resulting in a poor hydrodynamic efficiency to convert the shell kinetic energy into hot-spot pressure. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE{\-}NA0001944. [Preview Abstract] |
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