Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session UO7: Compression and Burn II |
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Chair: Jason Bates, Naval Research Laboratory Room: 203AB |
Thursday, October 26, 2017 2:00PM - 2:12PM |
UO7.00001: 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 ($h\nu =$ 1.865 keV) driven with 20-ps IR pulses from the OMEGA EP Laser System. The shadows of the converging DT ice shells have been obtained using a time-resolved (40-ps) narrowband crystal imaging system and improved backlight intensity. Measured x-ray radiographs are compared with their 1-D \textit{LILAC} and \textit{Spect3D }simulations. Moments of the imploded radial mass distributions are inferred from radiograph analysis based on Abel inversion. The sensitivity of the radiograph shadows to trace contamination by fuel--shell mix tests the hydrodynamic stability of the implosions indicated by their shell adiabats and in-flight aspect ratios. This work was supported by the U.S. Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 26, 2017 2:12PM - 2:24PM |
UO7.00002: Low-Mode Variations of the Cold-Fuel Distribution in Cryogenic DT Implosions on OMEGA C.J. Forrest, K.S. Anderson, V.Yu. Glebov, V.N. Goncharov, O.M. Mannion, P.B. Radha, S.P. Regan, T.C. Sangster, C. Stoeckl The neutron energy spectrum generated from cryogenic DT direct-drive implosions in inertial confinement fusion experiments is used to interpret the cold-fuel distribution at peak compression. At the Omega Laser Facility, measurements are used to extract the neutron spectrum utilizing a high-dynamic-range neutron time-of-flight spectrometer. The shape of the energy spectrum is fully determined by the neutron elastic scattering cross section for spherically symmetric target configurations. Significant differences from the expected shape have been measured for some recent implosions, which indicate a deviation from a spherically symmetric fuel assembly. Neutron scattering in the DT cold-fuel assembly has been modeled in radiation--hydrodynamic codes. The experimental data show reasonable agreement with the model when the mass distribution of the compressed DT shell has low-mode perturbations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 26, 2017 2:24PM - 2:36PM |
UO7.00003: Three-Dimensional Modeling of Low-Mode Asymmetries in OMEGA Cryogenic Implosions K.S. Anderson, P.W. McKenty, A. Shvydky, T.J.B. Collins, C.J. Forrest, J.P. Knauer, J.A. Marozas, F.J. Marshall, P.B. Radha, A.B. Sefkow, M.M. Marinak In direct-drive inertial confinement fusion implosions, long-wavelength asymmetries resulting from target offset, laser power imbalance, beam mispointing, etc. can be highly detrimental to target performance. Characterizing the effects of these asymmetry sources requires 3-D simulations performed in full-sphere geometry to accurately capture the evolution of shell perturbations and hot-spot flow. This paper will present 3-D \textit{HYDRA}\footnote{M. M. Marinak \textit{et al.}, Phys. Plasmas \textbf{8}, 2275 (2001).} simulations characterizing the impact of these perturbation sources on yield and shell modulation. Various simulated observables are generated, and trends are analyzed and compared with experimental data. This material is based on work supported by the Department of Energy National Nuclear Security Administration under Award Numbers DE-NA0001944 and performed under the auspices of the LLNL under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
Thursday, October 26, 2017 2:36PM - 2:48PM |
UO7.00004: Soft X-Ray Narrowband Radiography of Direct-Drive Cryogenic DT Implosions on OMEGA C. Stoeckl, R. Epstein, V.N. Goncharov, D.W. Jacobs-Perkins, R.K. Jungquist, C. Mileham, S.P. Regan, T.C. Sangster, W. Theobald Backlit images of cryogenic direct-drive implosions on OMEGA were recorded with a narrowband x-ray imager using an aspherically bent quartz crystal for the Si $\mbox{He}_{\alpha }^{\mathrm{\thinspace }}$line at $\sim $1.865 keV. These implosions are driven on a low adiabat (shell pressure/Fermi degenerate pressure), making them susceptible to Rayleigh--Taylor instabilities. The radiographic images can be used to study the performance of different shell materials like polystyrene and glow-discharge polymer with respect to small-scale mix from laser imprint, and long-wavelength variations of the compressed shell caused by target imperfections and laser illumination nonuniformities. The status of the radiography setup including work to improve the brightness of the backlighter, the alignment accuracy, and the spatial resolution of the imager will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 26, 2017 2:48PM - 3:00PM |
UO7.00005: Impact of flows on ion temperatures inferred from neutron spectra in asymmetrically driven OMEGA DT implosions M. Gatu Johnson, J. Frenje, B. Lahmann, F. Seguin, R. Petrasso, B. Appelbe, J. Chittenden, C. Walsh, J. Delettrez, I. Igumenshchev, J.P. Knauer, V.YU. Glebov, C. Forrest, W. Grimble, F. Marshall, T. Michel, C. Stoeckl, B.M. Haines, A.B. Zylstra Ion temperatures ($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 e.g. engineering features 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 OMEGA experiments with intentional drive asymmetry designed for testing the ability to accurately predict and measure line-of-sight differences in apparent $T_{\mathrm{ion}}$ due to low-mode asymmetry-seeded flows. The measurements are contrasted to CHIMERA, RAGE and ASTER simulations, providing insight into implosion dynamics and the relative importance of laser drive non-uniformity, stalk and offset as sources of asymmetry. The results highlight the complexity of hot-spot dynamics, which is a problem that must be mastered to achieve ICF ignition. This work was supported in part by the U.S. DOE, NLUF and LLE. [Preview Abstract] |
Thursday, October 26, 2017 3:00PM - 3:12PM |
UO7.00006: Testing the relative importance of ion diffusive transport and turbulent mixing with separated-reactant capsules Nelson Hoffman, Alex Zylstra Two recent capsule implosion shots at OMEGA, employing separated reactants (tritium gas surrounded by a layer of deuterated CD plastic) [A. B. Zylstra et al., in preparation], afford a simple test for distinguishing the importance of ion diffusive transport vs. turbulent mixing in the implosions. One capsule had a CD layer that was twice as thick as the other capsule: 0.3 $\mu $m vs 0.15 $\mu $m. Simulations using a turbulent-mix model together with an ion-diffusion model indicate that the thick-CD capsule would be expected to give higher DT yield than the thin-CD capsule, owing to the larger quantity of D available to mix with T. By contrast, simulations using the ion-diffusion model alone indicate that the thin-CD capsule would give the higher DT yield, owing to the fact that (a) it was driven somewhat harder than the thick-CD capsule and (b) only an extremely thin layer on the inside of the CD contributes significantly to the DT yield for either capsule, so the thickness difference is irrelevant. Measurements showed that in fact the thin-CD capsule gave higher DT yield, supporting the importance of ion diffusive transport in such capsule implosions. [Preview Abstract] |
Thursday, October 26, 2017 3:12PM - 3:24PM |
UO7.00007: Investigating the impact of species charge and mass on the manifestation of multi-ion and kinetic effects N. V. Kabadi, R. Simpson, H. Sio, M. Gatu Johnson, J. Frenje, B. Lahmann, C. Parker, R. D. Petrasso, C. Forrest, V. Yu Glebov, C. Stoeckl, S. Regan, H. G. Rinderknecht, G. Kagan Inertial confinement fusion implosions are almost exclusively modeled as hydrodynamic in nature, with a single average-ion fluid and fluid electrons. However, in the shock convergence phase of virtually all inertial fusion implosions, the mean-free path for ion-ion collisions becomes sufficiently long that both the shock front itself and the resulting central plasma are inadequately described by hydrodynamic modeling. In this regime individual ion species behave separately. Understanding how these multi-ion effects manifest themselves in both the kinetic and hydrodynamic regimes is of fundamental importance. In this presentation, first results from an investigation into the effects of individual species' mass and charge on multi-ion effects in kinetic and hydro-like regimes will be discussed. The work was supported by DOE, NLUF, LLNL and LLE. [Preview Abstract] |
Thursday, October 26, 2017 3:24PM - 3:36PM |
UO7.00008: Submicron-Scale Control of the Three-Dimensional Modes 1, 2, and 3 of Targets Imploded in the Direct-Drive Configuration on OMEGA D.T. Michel, I.V. Igumenshchev, A.K. Davis, D.H. Edgell, D.H. Froula, V.N. Goncharov, D.W. Jacobs-Perkins, S.P. Regan, A. Shvydky, E.M. Campbell Reducing low-mode nonuniformities has been identified as a critical step to demonstrate conditions for laser-direct-drive targets that are hydrodynamically equivalent to ignition when scaled to the megajoule energies at the National Ignition Facility. The 3-D shape of the imploding target was tomographically recorded using four lines-of-sight x-ray measurements of the ablation front. The projected ablation-front contours during the implosion phase were measured with framing cameras using the x-ray self-emission shadowgraphy technique. The projected ablation-front motions were obtained by comparing the positions of the contours on the framing cameras with the corresponding contour positions measured on a nonimploding solid CH ball shot. The amplitudes of the modes were determined within \textpm 0.15{\%} by decomposition into spherical harmonics of the contours oriented perpendicular to the lines-of-sight and shifted by the measured motions. The variations of the amplitudes in modes 1, 2, and 3 between shots were shown to change linearly (within $\pm $0.25{\%}) with the variations of the mode amplitudes of the laser beam energy balance making it possible to compensate the residual target modes (that remain when the laser is balanced) within 1{\%}. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 26, 2017 3:36PM - 3:48PM |
UO7.00009: Impacts of Implosion Asymmetry And Hot Spot Shape On Ignition Capsules Baolian Cheng, Thomas J.T. Kwan, Yi-Ming Wang, S. Austin Yi, Steve Batha Implosion symmetry plays a critical role in achieving high areal density and internal energy at stagnation during hot spot formation in ICF capsules. Asymmetry causes hot spot irregularity and stagnation de-synchronization that results in lower temperatures and areal densities of the hot fuel. These degradations significantly affect the alpha heating process in the DT fuel as well as on the thermonuclear performance of the capsules. In this work, we explore the physical factors determining the shape of the hot spot late in the implosion and the effects of shape on α-particle transport. We extend our ignition theory [1-4] to include the hot spot shape and quantify the effects of the implosion asymmetry on both the ignition criterion and capsule performance. We validate our theory with the NIF existing experimental data Our theory shows that the ignition criterion becomes more restrictive with the deformation of the hot spot. Through comparison with the NIF data, we demonstrate that the shape effects on the capsules’ performance become more explicit as the self-heating and yield of the capsules increases. The degradation of the thermonuclear burn by the hot spot shape for high yield shots to date can be as high as 20%. Our theory is in good agreement with the NIF data. [Preview Abstract] |
Thursday, October 26, 2017 3:48PM - 4:00PM |
UO7.00010: Effects of Hot-Spot Geometry on Backscattering and Down-Scattering Neutron Spectra Z.L. Mohamed, O.M. Mannion, C.J. Forrest, J.P. Knauer, K.S. Anderson, P.B. Radha The measured neutron spectrum produced by a fusion experiment plays a key role in inferring observable quantities. One important observable is the areal density of an implosion, which is inferred by measuring the scattering of neutrons. This project seeks to use particle-transport simulations to model the effects of hot-spot geometry on backscattering and down-scattering neutron spectra along different lines of sight. Implosions similar to those conducted at the Laboratory of Laser Energetics are modeled by neutron transport through a DT plasma and a DT ice shell using the particle transport codes \textit{MCNP} and \textit{IRIS}. Effects of hot-spot geometry are obtained by ``detecting'' scattered neutrons along different lines of sight. This process is repeated for various hot-spot geometries representing known shape distortions between the hot spot and the shell. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 26, 2017 4:00PM - 4:12PM |
UO7.00011: Determining hot spot motion using a multi line-of-sight nToF analysis Robert Hatarik, Ryan Nora, Brian Spears, Mark Eckart, Edward Hartouni, Gary Grim, Alastair Moore, David Schlossberg An important diagnostic value of a shot at the National Ignition Facility (NIF) is the resultant center-of mass motion of the imploding capsule as it contributes to the efficiency of converting LASER energy into plasma temperature. In the past the projection of this velocity onto a line-of-sight (LOS) for a given detector was determined by using a temperature model to determine the mean nergy of the emitted neutrons. With the addition of a fourth neutron time-of-flight LOS at the NIF, it is possible to determine a hot spot vector and mean velocity of the emitted neutron distribution. This entails analyzing all four LOS simultaneously and has the advantage of not relying on a temperature model. Results from recent NIF shots comparing this method with the traditional method will be presented. [Preview Abstract] |
Thursday, October 26, 2017 4:12PM - 4:24PM |
UO7.00012: Implosion anisotropy of neutron kinetic energy distributions as measured with the neutron time-of-flight diagnostics at the National Ignition Facility Edward Hartouni, Mark Eckart, John Field, Gary Grim, Robert Hatarik, Alastair Moore, David Munro, Daniel Sayer, David Schlossberg Neutron kinetic energy distributions from fusion reactions are characterized predominantly by the excess energy, Q, of the fusion reaction and the variance of kinetic energy which is related to the thermal temperature of the plasma as shown by \emph{e.g.} Brysk\footnote{H. Brysk, \emph{Plasma Phys.,} {\bf 15} 611 (1973)}. High statistics, high quality neutron time-of-flight spectra obtained at the National Ignition Facility provide a means of measuring small changes to the neutron kinetic energy due to the spatial and temporal distribution of plasma temperature, density and velocity. The modifications to the neutron kinetic energy distribution as described by Munro\footnote{D. H. Munro, \emph{Nucl. Fusion,} {\bf 56} (2016) 036001} include plasma velocity terms with spatial orientation, suggesting that the neutron kinetic energy distributions could be anisotropic when viewed by multiple lines-of-sight. These anisotropies provide a diagnostic of burn averaged plasma velocity distributions. We present the results of measurements made for a variety of DT implosions and discuss their possible physical interpretations. [Preview Abstract] |
Thursday, October 26, 2017 4:24PM - 4:36PM |
UO7.00013: Precision Neutron Time-of-Flight Detectors Provide Insight into NIF Implosion Dynamics David Schlossberg, M.J. Eckart, G.P. Grim, E.P. Hartouni, R. Hatarik, A.S. Moore, C.S. Waltz During inertial confinement fusion, higher-order moments of neutron time-of-flight (nToF) spectra can provide essential information for optimizing implosions. The nToF diagnostic suite at the National Ignition Facility (NIF) was recently upgraded to include novel, quartz Cherenkov detectors. These detectors exploit the rapid Cherenkov radiation process, in contrast with conventional scintillator decay times, to provide high temporal-precision measurements that support higher-order moment analyses. Preliminary measurements have been made on the NIF during several implosions and initial results are presented here. Measured line-of-sight asymmetries, for example in ion temperatures, will be discussed. Finally, advanced detector optimization is shown to advance accessible physics, with possibilities for energy discrimination, gamma source identification, and further reduction in quartz response times. [Preview Abstract] |
Thursday, October 26, 2017 4:36PM - 4:48PM |
UO7.00014: Measurements of Ion Stopping around the Bragg Peak and its dependence on electron temperature and density in High-Energy-Density Plasmas (HEDP) J. Frenje, C.K. Li, F. Seguin, M. Gatu Johnson, H. Sio, R. Petrasso, T. Nagayama, R. Mancini, R. Hernandez, P. Grabowski, H.G. Rinderknecht, V. Yu Glebov Ion stopping around the Bragg peak and its dependence on plasma conditions were recently measured for the first time in HEDP [1]. The data support most stopping-power models for ion velocities (v$_{\mathrm{i}})$ larger than the average velocity of the thermal electrons (v$_{\mathrm{th}})$, but there are some differences at v$_{\mathrm{i}}$\textasciitilde v$_{\mathrm{th}}$, which could not be fully explored. This work described here makes significant advances over the first experimental effort by quantitatively assessing the characteristics of ion stopping around the Bragg peak and its dependence on electron temperature and density in HEDP. This effort represents the first sensitive test of plasma-stopping-power models around the Bragg peak, which is an important first step in our efforts to obtain a fundamental understanding of DT-alpha stopping in HEDP, a prerequisite for understanding ignition margins in various implosion designs. The work was supported by DOE, NLUF, LLNL and LLE. [1] Frenje et al., PRL (2015). [Preview Abstract] |
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