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 UO5: Stagnation II |
Hide Abstracts |
Chair: Austin Yi, Los Alamos National Laboratory Room: 230 B |
Thursday, November 3, 2016 2:00PM - 2:12PM |
UO5.00001: Achievement of Core Conditions for Alpha Heating in DirectÂDrive Inertial Confinement Fusion A. Bose, K.M. Woo, R. Betti, D. Mangino, A.R. Christopherson, W. Theobald, E.M. Campbell, R.L. McCrory, S.P. Regan, V.N. Goncharov, T.C. Sangster, C.J. Forrest, V.Yu. Glebov, J.P. Knauer, F.J. Marshall, C. Stoeckl, R. Nora, J.A. Frenje, M. Gatu Johnson, D. Shvarts It is shown for the first time that direct-drive implosions on the OMEGA laser have achieved core conditions that would lead to significant alpha heating at incident energies available at the National Ignition Facility (NIF) scale. The extrapolation of the experimental results from OMEGA to NIF energy assumes only that the implosion hydrodynamic efficiency is unchanged at higher energies. This approach is independent of the uncertainties in the physical mechanism that degrade implosions on OMEGA, and relies solely on a volumetric scaling of the experimentally observed~core conditions. It is estimated that the current best-performing OMEGA implosion extrapolated to a 1.9-MJ laser driver with the same illumination configuration and laser--target coupling would produce 125 kJ of fusion energy with similar levels of alpha heating observed in current highest performing indirect-drive NIF implosions. This conclusion is reached using an analytic scaling as well as direct numerical simulations of energy-scaled targets. 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 2:12PM - 2:24PM |
UO5.00002: Understanding scaling of ignition metrics for high-yield implosions on the NIF Paul Springer, Omar Hurricane, J. H. Hammer, D. A. Callahan, D. T. Casey, C. J. Cerjan, M. J. Edwards, J. E. Field, j. Gaffney, G. P. Grim, A.L. Kritcher, T. Ma, A. G. MacPhee, D. H. Munro, R. C. Nora, P. K. Patel, L. Peterson, B. Spears The self-heating condition for an imploding hotspot requires understanding the balance between mechanical work, heating via fusion reactions, and the radiative and conduction losses. A 3D cognizant Lawson ignition threshold metric is derived based on net fusion hotspot heating achieved when hotspot rho-r and ion temperature exceed critical values that depend on the temperature-dependent loss mechanisms. Key to understanding and scaling such analysis is an accurate determination of hotspot density and pressure, which are generally inferred using the yield, the thermal temperature, and other experimental data. 3D flow and its effect on neutron spectra can lead to overestimation of the temperature, and underestimation of hotspot rho-r, energy, and ignition margin. In this work, we analyze these effects in NIF data, and propose new methods to avoid them. These simple, analytical methods are tested using the largest 2D ICF simulation dataset ever produced. [Preview Abstract] |
Thursday, November 3, 2016 2:24PM - 2:36PM |
UO5.00003: Quantifying uncertainty in NIF implosion performance across target scales$\backslash $f1 Brian Spears, K. Baker, S. Brandon, M. Buchoff, D. Callahan, D. Casey, J. Field, J. Gaffney, J. Hammer, K. Humbird, O. Hurricane, M. Kruse, D. Munro, R. Nora, L. Peterson, P. Springer, C. Thomas Ignition experiments at NIF are being performed at a variety of target scales. Smaller targets require less energy and can be fielded more frequently. Successful small target designs can be scaled up to take advantage of the full NIF laser energy and power. In this talk, we will consider a rigorous framework for scaling from smaller to larger targets. The framework uses both simulation and experimental results to build a statistical prediction of target performance as scale is increased. Our emphasis is on quantifying uncertainty in scaling predictions with the goal of identifying the dominant contributors to that uncertainty. We take as a particular example the Big Foot platform that produces a round, 0.8 scale implosion with the potential to scale to full NIF size (1.0 scale). [Preview Abstract] |
Thursday, November 3, 2016 2:36PM - 2:48PM |
UO5.00004: On the importance of minimizing ``coast-time'' in x-ray driven inertial confinement fusion implosions O.A. Hurricane, D.A. Callahan, D.T. Casey, E.L. Dewald, T.R. Dittrich, T. Doeppner, D.E. Hinkel, L.F. Berzak Hopkins, A. Kritcher, O. Landen, S. Le Pape, T. Ma, A. MacPhee, A. Pak, H.-S. Park, P.K. Patel, J. Ralph, J.D. Salmonson, P.T. Springer By the time an ICF implosion has converged a factor of 20, its surface area has shrunk 400x, making it an inefficient x-ray energy absorber. So traditionally, ICF implosions are designed to have the laser drive shut off at a time, $t_{off}$, well before bang-time, $t_{BT}$, for a coast-time of $t_{coast}=t_{BT}-t_{off}.$ Contrary to expectations, high-foot implosions on NIF show a strong dependence of many key ICF quantities on reduced coast-time (by extending the duration of laser peak power at constant power), most notably stagnation pressure. Herein we show that the ablation pressure, $p_{abl}$, which drives high-foot implosions, is essentially triangular in temporal shape, and that reducing $t_{coast}$ boosts $p_{abl}$ by \textasciitilde 2x. Analytic theory demonstrates that reducing coast-time can lead to a \textasciitilde 15{\%} higher implosion velocity, which together with the increased ablation pressure, can boost the stagnation pressure by \textasciitilde 2x as compared to a coasting version of the same implosion. Four dimensionless parameters are identified. We find that reducing coast-time to as little as 500 ps still provides some benefit. [Preview Abstract] |
Thursday, November 3, 2016 2:48PM - 3:00PM |
UO5.00005: Using absolute x-ray spectral measurements to infer stagnation conditions in ICF implosions Pravesh Patel, L. R. Benedetti, C. Cerjan, D. S. Clark, O. A. Hurricane, N. Izumi, L. C. Jarrott, S. Khan, A. L. Kritcher, T. Ma, A. G. MacPhee, O. Landen, B. K. Spears, P. T. Springer Measurements of the continuum x-ray spectrum emitted from the hot-spot of an ICF implosion can be used to infer a number thermodynamic properties at stagnation including temperature, pressure, and hot-spot mix. In deuterium-tritium (DT) layered implosion experiments on the National Ignition Facility (NIF) we field a number of x-ray diagnostics that provide spatial, temporal, and spectrally-resolved measurements of the radiated x-ray emission. We report on analysis of these measurements using a 1-D hot-spot model to infer thermodynamic properties at stagnation. We compare these to similar properties that can be derived from DT fusion neutron measurements. 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 3:00PM - 3:12PM |
UO5.00006: Compression versus first shock strength in indirect-drive NIF implosions Otto Landen, Peter Celliers, Harry Robey, Laura Berzak Hopkins, Steve Haan, John Lindl NIF indirect-drive cryogenic DT implosions have used a variety of multi-shock pulse shapes to implode capsules with in-flight fuel adiabats$^{\mathrm{1}}$ ranging from 1.5 to 4. At a given design adiabat, the stagnated convergence ratio and fuel areal density inferred from the neutron image size and the ratio of downscattered to primary neutron yield shows variability that can be ascribed to shot-to-shot differences in shock timing, ablator dopant level and duration of coast phase. However, the locus of maxima in convergence and fuel areal density is shown to depend principally on the first shock strength that is measured by separate shock timing shots. No clear secondary dependence on hot electron preheat levels that vary by orders of magnitude between designs is observed. The scalings, which include all NIF indirect-drive implosions shot to date, are fitted using an analytic 1D implosion model$^{\mathrm{2}}$. $^{\mathrm{1}}$H.F. Robey \textit{et. al}., Phys. Plasmas 23, 056303 (2016). $^{\mathrm{2}}$C.D. Zhou and R. Betti, Phys. Plasmas 14, 072703 (2007). [Preview Abstract] |
Thursday, November 3, 2016 3:12PM - 3:24PM |
UO5.00007: Simulated impact of self-generated magnetic fields in the hot-spot of NIF implosions M. A. Partha, S. W. Haan, J. Koning, M. M. Marinak, C. R. Weber, D. S. Clark Deviations from sphericity in an imploded hot-spot result in magnetic fields generated by the Biermann battery effect. The magnetic field can reduce thermal conductivity, affect $\alpha $ transport, change instability growth, and cause magnetic pressure. Previous estimates of these effects have indicated that they are not of great consequence, but have suggested that they could plausibly affect NIF observables such as yield and ion temperature by 5-25{\%}. Using the MHD capability in the Hydra code, we evaluated the impact of these processes in a post-shot model for a typical NIF implosion. Various implosion asymmetries were implemented, with the goal of surveying plausible implosion configurations to find the geometry in which the MHD effects were the most significant. Magnetic fields are estimated to approach 10$^{\mathrm{4}}$ Tesla, and to affect conductivity locally by more than 50{\%}, but global impact on observables is small in most cases. *Work performed under the auspices of the U.S. D.O.E. by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
Thursday, November 3, 2016 3:24PM - 3:36PM |
UO5.00008: Self-Generated Magnetic Fields in Stagnation-Phase ICF Implosions. Christopher Walsh, Jeremy Chittenden, Kristopher McGlinchey, Nicolas Niasse 3-D extended-MHD simulations of the stagnation phase of an ICF implosion are presented, showing significant self-generated magnetic fields (1000-5000T) due to the Biermann Battery effect. Perturbed hot-spots generate magnetic fields at their edges, as the extremities of hot bubbles are rapidly cooled by the surrounding low temperature fuel, giving non-parallel electron pressure and density gradients. Larger amplitude and higher mode-number perturbations lead to an increased hot-spot surface area and more heat flow, developing greater non-parallel gradients and therefore larger magnetic fields. Due to this, largely perturbed hot-spots can be affected more by magnetic fields, although the accelerated cooling associated with greater deviations from symmetry lowers magnetisation. The Nernst effect advects magnetic field down temperature gradients towards the outer region of the hot-spot, which can also lower the magnetisation of the plasma. In some regions, however, the Nernst velocity is convergent, magnetising the tips of cold fuel spikes, resulting in anisotropic heat-flow and an improvement in energy containment. Low-mode and multi-high-mode simulations are shown, with magnetisations reaching sufficiently high levels in some regions of the hot-spot to suppress thermal conduction to lower than 50{\%} of the unmagnetised case. A quantitative analysis of how this affects the hot-spot energy balance is included. [Preview Abstract] |
Thursday, November 3, 2016 3:36PM - 3:48PM |
UO5.00009: ABSTRACT WITHDRAWN |
Thursday, November 3, 2016 3:48PM - 4:00PM |
UO5.00010: Non-equilibrium electron features in X-ray emission spectrum from inertial confinement fusion implosions Grigory Kagan, O. L. Landen, D. Svyatsky, D. Thorn, M. B. Schneider, D. Bradley, J. D. Kilkenny An X-ray spectrometer proposed for the National Ignition Facility will infer the imploded core electron temperature from the free-free continuum spectra of the emitted photons with energies of 15 to 30 keV. In this range reabsorption rates are low so one might expect a rather unambiguous temperature measurement from the spectrum slope at the higher energy cut-off. It can be noticed, however, that the harder X-ray radiation is emitted by the tail of the electron distribution. The mean- free-path for the suprathermal electrons is much larger than for their thermal counterparts, making this tail to deviate from Maxwellian and obscuring interpretation of the data. We utilize solutions for the reduced kinetic equation to investigate modification to the X-ray spectra due to suprathermal electrons’ deviation from equilibrium. The logarithmic slope of the spectrum from the depleted electron distribution is found to increasingly drop at higher photon energies compared to the case of perfectly Maxwellian electrons. Interpreting the spectrum from a depleted distribution with assumption of Maxwellian electrons enforced gives the electron temperature lower than the actual one. The newly predicted effects are further enhanced in the presence of hydrodynamic mix. [Preview Abstract] |
Thursday, November 3, 2016 4:00PM - 4:12PM |
UO5.00011: Integrative Analysis of Hot Spot Conditions in MagLIF Experiments Patrick Knapp, Matthew Gomez, Eric Harding, Stephanie Hansen, Kelly Hahn, Matthias Geissel, Gordon Chandler, Ian Smith, Steve Slutz, Chris Jennings, Matthew Martin, Paul Schmit, Kyle Peterson, Gregory Rochau, Ryan McBride, Daniel Sinars A large data set incorporating all available neutron and x-ray data is used to analyze a broad range of Magnetized Liner Inertial Fusion (MagLIF) experiments conducted on the Z machine at Sandia National Laboratories over the past two years. Electron and ion temperatures, electron density, mix fraction, burn volume and duration, and neutron and x-ray yields are all measured on each experiment; several through multiple independent methods. Complementary methods are used to infer the hot spot energy and pressure, and trends are analyzed. The results are placed in the context of accepted performance metrics for Magneto-Inertial Fusion. [Preview Abstract] |
Thursday, November 3, 2016 4:12PM - 4:24PM |
UO5.00012: Modification of stagnation conditions in Magnetized Liner Inertial Fusion via thick dielectric coating M. R. Gomez, E. C. Harding, K. J. Peterson, T. J. Awe, D. J. Ampleford, S. B. Hansen, C. A. Jennings, M. R. Weis, G. A. Chandler, K. D. Hahn, P. F. Knapp, M. R. Martin, R. D. McBride, G. A. Rochau, A. B. Sefkow, D. B. Sinars, E. P. Yu Magnetized Liner Inertial Fusion (MagLIF) experiments on the Z facility at Sandia National Laboratories use approximately 20 MA of current to implode a metal cylinder, which contains axially-magnetized, laser-heated deuterium fuel. MagLIF experiments have demonstrated primary DD neutron yields up to 3e12 with burn averaged ion temperatures of 2.5 keV. X-ray emission at stagnation, recorded with a spherically-bent crystal imager, shows a weakly-helical structure with axial variations in intensity. Previously, the application of a thick dielectric coating to the exterior of an imploding cylinder has shown improved stability of the cylinder throughout the implosion. We recently demonstrated that adding a dielectric coating to a MagLIF target produces a cylindrical, rather than helical, stagnation column with reduced axial variations in intensity. There are also indications of decreased late-time mix in the x-ray spectra. This is consistent with a more uniform, stable stagnation column. *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Thursday, November 3, 2016 4:24PM - 4:36PM |
UO5.00013: Differentiating Different Modeling Assumptions in Simulations of MagLIF loads on the Z Generator C.A. Jennings, M.R. Gomez, E.C. Harding, P.F. Knapp, D.J. Ampleford, S.B. Hansen, M.R. Weis, M.E. Glinsky, K. Peterson, J.P. Chittenden Metal liners imploded by a fast rising (\textless 100ns) current to compress a magnetized, preheated fuel offer the potential to efficiently reach fusion conditions [S.A. Slutz et al. Phys. Plasmas 17, 056303 (2010)]. These MagLIF experiments have had some success [M.R. Gomez et al Phys. Rev. Lett. 113, 155003(2014)]. While experiments are increasingly well diagnosed, many of the measurements (particularly during stagnation) are time integrated, limited in spatial resolution or require additional assumptions to interpret in the context of a structured, rapidly evolving system. As such, in validating MHD calculations, there is the potential for the same observables in the experimental data to be reproduced under different modeling assumptions. Using synthetic diagnostics of the results of different pre-heat, implosion and stagnation simulations run with the Gorgon MHD code, we discuss how the interpretation of typical Z diagnostics relate to more fundamental simulation parameters. We then explore the extent to which different assumptions on instability development, current delivery, high-Z mix into the fuel and initial laser deposition can be differentiated in our existing measurements. [Preview Abstract] |
Thursday, November 3, 2016 4:36PM - 4:48PM |
UO5.00014: Ignition conditions relaxation for central hot-spot ignition with an ion-electron non-equilibrium model Zhengfeng Fan, Jie Liu We present an ion-electron non-equilibrium model, in which the hot-spot ion temperature is higher than its electron temperature so that the hot-spot nuclear reactions are enhanced while energy leaks are considerably reduced. Theoretical analysis shows that the ignition region would be significantly enlarged in the hot-spot rhoR-T space as compared with the commonly used equilibrium model. Simulations show that shocks could be utilized to create and maintain non-equilibrium conditions within the hot spot, and the hot-spot rhoR requirement is remarkably reduced for achieving self-heating. In NIF high-foot implosions, it is observed that the x-ray enhancement factors are less than unity, which is not self-consistent and is caused by assuming Te$=$Ti. And from this non-consistency, we could infer that ion-electron non-equilibrium exists in the high-foot implosions and the ion temperature could be \textasciitilde 9{\%} larger than the equilibrium temperature. [Preview Abstract] |
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