Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session UO4: Direct and Indirect Drive |
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Chair: Laurent Masse, Lawrence Livermore National Laboratory Room: OCC B110-112 |
Thursday, November 8, 2018 2:00PM - 2:12PM |
UO4.00001: Toward a burning plasma state using high density carbon ablator inertially confined fusion implosions on the National Ignition Facility Sebastien Le Pape, Laura Berzak Hopkins, Laurent Divol, Arthur Pak, Eduard L. Dewald, Darwin Ho, Clement S Goyon, Debra Ann Callahan, Omar A Hurricane In indirect-drive Inertial Confinement Fusion (ICF), a spherical shell of cryogenic deuterium- tritium (DT) fuel surrounded by a low Z ablator is imploded at high velocity. The kinetic energy of the shell (fuel + unablated material) is converted at stagnation to internal energy of a central hot-spot. Recent experiments on the National Ignition Facility (NIF) using High Density Carbon ablators generated 54 kJ of fusion output, exceeding for the first time the peak kinetic energy of the shell by more than a factor of 2. Implosions have achieved hot spot areal densities rR » 0.3 g/cm3 and stagnation pressures P = 360 Gbar, higher than at the center of the sun. The fraction of alpha particles stopped in the hot spot » 85 % is enough to sustain self-heating once the number of D-T reactions is sufficient. Current experiments are investigating hot spot physics in the high alpha regime, including sensitivity to velocity, symmetry, and shock-timing. The results of current experiments with 980 μm outer radius capsules will inform future implosions using larger HDC capsules at similar adiabat and peak velocity. |
Thursday, November 8, 2018 2:12PM - 2:24PM |
UO4.00002: Analysis of NIF implosion experiment N170601 T. R. Dittrich, D. S. Clark, C. R. Weber, L. F. Berzak Hopkins, A. E. Pak, D. D. Ho, J. A. Harte, G. B. Zimmerman NIF implosion experiment N170601 produced one of the highest neutron outputs observed to date (1.46E16 primary neutrons). This High Density Carbon (HDC) ablator capsule design had a 5 micron outside diameter glass fill tube attached. Post shot simulations indicate the hydrodynamic jet formed from this tube during capsule implosion may have inhibited even better performance. Observations from this experiment include DT fusion yield, bang time, DSR, Tion and a time-integrated x-ray emission image. Several modifications to N170601 will be presented that attempt to improve the performance of the implosion. |
Thursday, November 8, 2018 2:24PM - 2:36PM |
UO4.00003: BigFoot program and projections: the basics of indirect drive ICF Cliff A Thomas We report a set of DT layered implosions used to test and quantify the experimental tradeoffs between velocity, coast, and adiabat at two physical scales. The laser power and energy were adjusted to maintain implosion symmetry at velocities from 330 to 430 um/ns, at adiabats 3 and 4, at a “coast time” from 700 ps to 2 ns (the time between peak radiation temperature in the hohlraum and capsule stagnation). For all tests the performance is found to be monotonic and consistent with simple theory, except in the case of adiabat. The highest adiabat configuration (4) resulted in the highest neutron yield to-date at the National Ignition Facility (NIF), and can be used to project the energy and power requirements for ignition [1]. |
Thursday, November 8, 2018 2:36PM - 2:48PM |
UO4.00004: Performance tests of implosion velocity, coast, and adiabat using the Bigfoot platform on the National Ignition Facility (NIF) Kevin Baker, Cliff A Thomas, Daniel T Casey, Matthias Hohenberger, Shahab Khan, Annie Kritcher, Tod Woods, Ryan Nora, Brian K. Spears, David Munro, Jose L Milovich, Richard L Berger, David Jerome Strozzi, Charles B Yeamans, Robert Hatarik, Maria Gatu Johnson, Tammy Yee Wing Ma, Laura Robin Benedetti, Nobuhiko Izumi, Omar A Hurricane, Debra Ann Callahan For indirect drive inertial confinement fusion a spherical shell of DT fuel is imploded to reach the high density and temperature conditions needed for fusion. The Bigfoot approach uses a 3 shock laser pulse with a short foot, with the second shock overtaking the first in the ablator, to place the DT fuel on a high adiabat (~4). To understand the experimental tradeoffs between velocity and adiabat, the Bigfoot design was employed to study changes in velocity and adiabat in a series of experiments. These experiments varied the laser power and energy to test implosion performance with velocities from ~320 - 430 km/s and varied the length of the trough to access adiabats from 3 – 4. The impact of a 25% reduction in “coast” was also demonstrated. This set of experiments demonstrated low-mode symmetry control and performance that follows simple scalings for symmetry and performance. This high adiabat approach has led to the highest DT neutron yield achieved thus far on the NIF. The results of a scan in adiabat will also be presented. The results from this design are feeding into future designs for ignition on NIF.
*This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. |
Thursday, November 8, 2018 2:48PM - 3:00PM |
UO4.00005: Developing the Hybrid-B campaign for high laser energy and power at the National Ignition Facility Daniel T Casey, Andrea Kritcher, Cliff A Thomas, Matthias Hohenberger, Kevin L Baker, Sebastien Le Pape, Daniel Clark, Christopher Weber, Michael Stadermann, Juergen Biener, Abbas Nikroo, Arthur Pak, Laurent Divol, Jose L Milovich, David Jerome Strozzi, Richard L Berger, Douglas T Woods, Brandon Nathan Woodworth, James Sevier, Michael J Edwards, Debra Ann Callahan, Omar A Hurricane, Alex Zylstra To achieve hotspot ignition, inertial confinement fusion implosions must achieve high hotspot pressures that are inertially confined by a dense shell of DT fuel. Recent experiments at the National Ignition Facility (NIF) have shown improvements in implosion performance with increases in coupled drive/shell energy, velocity, and overall scale. To better understand these experiments, a common model was used [1] to compare HDC, CH, and Be based implosions and subsequently to develop a “hybrid” design based on many of their most successful design features. The first of these Hybrid concepts (Hybrid-B) was designed to use very high laser power and energy at the NIF with the largest capsule scale where good hotspot/drive coupling, including symmetry control, can be maintained. Operating near the current limits of the NIF further constrains the design options for maintaining implosion low mode symmetry. Therefore, the Hybrid-B experimental campaign was conducted with two hohlraum gas-fills to develop a platform enabling future experiments at the highest energies ~2.1 MJ yet attempted at the NIF.
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Thursday, November 8, 2018 3:00PM - 3:12PM |
UO4.00006: HybridB design and comparison to data Andrea Kritcher, Cliff A Thomas, Daniel T Casey, Matthias Hohenberger, Kevin L Baker, Sebastien Le Pape, Daniel Clark, Christopher Weber, Michael Stadermann, Juergen Biener, A. Nikroo, Arthur Pak, Laurent Divol, Jose L Milovich, David Jerome Strozzi, Richard L Berger, Douglas T Woods, Brandon Nathan Woodworth, James Sevier, Michael J Edwards, Debra Ann Callahan, Omar A Hurricane, Alex Zylstra, N. Rice, Casey Kong The maximal performance of Inertial Confinement Fusion (ICF) implosions is governed by four key parameters, implosion velocity, adiabat, inflight ablation pressure and capsule size [1]. Experiments on the NIF have roughly bracketed the values of implosion velocity, adiabat and ablation pressure for current systems. Using these parameters, “hybrid” campaigns now aim to optimize the capsule size that produces overall optimal performance in a cylindrical hohlraums at full NIF power and energy. An experimental database was used to inform hohlraum design choices [2] to preserve symmetry. The first of these campaigns, “HybridB”, used a high density carbon (HDC or diamond) ablator and combined features of current platforms and new aspects to increase capsule scale by 5-15%. Results show sufficient inner beam propagation and a path to further increasing the capsule scale. This talk outlines the HybridB design and comparison to experimental data. [1] O. A. Hurricane, in prep (2018). |
Thursday, November 8, 2018 3:12PM - 3:24PM |
UO4.00007: Experimental Results from DT Layered Alternate Tent Support Implosions on the NIF Joseph E Ralph, Tilo Doeppner, Bruce A Hammel, Vladimir Smalyuk, Christopher Weber, Denise E Hinkel, Michael Stadermann, Arthur Pak, Debra Ann Callahan, Andrea Kritcher, Omar A Hurricane, S. Austin Yi, Alex Zylstra Engineering features on the capsule remain of great concern for ICF implosions on the NIF. Other than the fill tube, the tent remains as the engineering feature with the greatest potential for seeding Rayleigh-Taylor instabilities. Simulations indicate that the current 3-shock high foot CH ablator designs are most sensitive to perturbations produced by the tent. Experiments additionally have shown that the tent is causing a significant shell perturbation. Recently, a cryogenic layered DT implosion experiment was performed in which only the capsule support was changed so that the capsule tent support contact area has been minimized. Results from this experiment and supporting experiments will be presented. This experiment is the first CH experiment to demonstrate total neutron yield at or exceeding 1x1016 despite numerous attempts with the same exact laser pulse and target, but with the standard tent capsule support. When compared with prior alternate tent implosion experiments, this experiment may also indicate that the benefit on the alternate tent support is significantly greater when using a thinner ablator. |
Thursday, November 8, 2018 3:24PM - 3:36PM |
UO4.00008: The DT to DD yield ratio as a diagnostic of implosion dynamics for NIF ICF experiments. Edward P Hartouni, Richard Marshall Bionta, Mark J. Eckart, John E Field, Maria Gatu Johnson, Gary P Grim, Robert Hatarik, Shaun Kerr, Joe Kilkenny, David Munro, Alastair S Moore, David J. Schlossberg The variation of the yield ratio YDT/YDD for implosions of cryogenically layered deuterium and tritium (DT) fueled capsules exceeds the expectation based on measurement statistical uncertainties and fuel isotopic concentration [M. Gatu-Johnson, et al., Phys. Plasmas, 20 (2013) 042707]. Variations in the line-of-sight diagnostics depend on both the attenuation of the "birth spectrum" of the neutrons through the compressed, cold fuel and the burn-averaged moments of the fuel velocity distributions [D. H. Munro, Nucl. Fusion, 56 (2016) 036001]. An analysis of the variability of the yield ratio for recent implosions sets limits on the size of the velocity contributions. These limits are compared to expectations from 2D and 3D simulation ensembles of similar implosions. |
Thursday, November 8, 2018 3:36PM - 3:48PM |
UO4.00009: In-Situ Measurements of Direct-Drive Illumination Uniformity on OMEGA Frederic J Marshall, Valeri N Goncharov, John Kelly, Tanya Kosc, Alexander Shvydky Experiments performed on the OMEGA Laser System have measured the 60-beam, direct-drive illumination uniformity. The UV illumination variations are inferred from images of x-ray emission from Au-coated spherical targets. The experiments are performed with spheres having diameters ranging from 800 to 980 microns, mounted on stalks identical to those used on cryogenic target experiments (17-micron-diam SiC fibers), and are diagnosed with an array of 11 digitally recorded x-ray pinhole camera images that sample the entire surface flux of x rays emitted by the target. Both 100-ps and 1-ns pulses are used in these experiments. The various effects that are important to the direct-drive, low-mode illumination uniformity (l < 30) such as beam overlap, target positioning, and stalk shadowing are all well measured with this method. |
Thursday, November 8, 2018 3:48PM - 4:00PM |
UO4.00010: Dependence of Hot-Spot Mix in DT Cryogenic Implosions on the Design Adiabat Sean P Regan, Valeri N Goncharov, Reuben Epstein, Riccardo Betti, Mark J Bonino, Duc M Cao, Timothy J Collins, Edward Michael Campbell, Chad Forrest, Vladimir Glebov, David R Harding, James P Knauer, John A Marozas, Frederic J Marshall, P.B Radha, Thomas C Sangster, Rahul C Shah, Christian Stoeckl, Rain W Luo, Michael E Schoff, Michael Farrell The measured dependence of hot-spot mix[1] on the design adiabat (a = Pshell/PFermi) for laser direct drive (LDD) implosions of DT cryogenic targets on the 60-beam, 30-kJ, 351-nm OMEGA laser is presented. The adiabat of the implosion was controlled by adjusting the temporal shape of the laser drive pulse. Perturbations seeded by debris, target imperfections, engineering features, such as the stalk or fill tube, and laser imprint are amplified by the Richtmyer–Meshkov instability during the shock transit of the shell and by the Rayleigh–Taylor instability of the ablation front during the acceleration phase.[2] The mixing of Ge-doped plastic ablator material with the interior DT fuel was diagnosed with x-ray spectroscopy at stagnation.1 [1] S. P. Regan et al., Phys. Rev. Lett. 111, 045001 (2013). [2] I. V. Igumenshchev et al., Phys. Plasmas 20, 082703 (2013).
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Thursday, November 8, 2018 4:00PM - 4:12PM |
UO4.00011: Arresting Sound Waves to Mitigate Laser Imprint in Direct-Drive Implosions I. V. Igumenshchev, R. C. Shah, R. Betti, E. M. Campbell, V. N. Goncharov, J. P. Knauer, S. P. Regan, A. Shvydky, A. L. Velikovich, A. J. Schmitt Performance of direct-drive implosions can suffer from laser imprint, which introduces broadband modulations in implosion shells. The effects of imprint in OMEGA implosions were studied in three dimensions using the code ASTER. It is shown that the most destructive short-wavelength imprint modulations (with Legendre modes l > 30) are seeded at the outer edge of target shells during the first 100 ps of the laser pulse. Implosions driven by laser pulses with pickets produce aftershock rarefaction flows, which can arrest sound waves propagating inward and, therefore, can prevent the development of imprint modulations inside the shell by these waves. Implosions driven by continuous pulses do not produce such flows and sound waves can propagate inward and develop imprint modulations. ASTER simulations suggest that an optimum choice of laser pulses and target dimensions can reduce the development of short-wavelength modulations and, therefore, mitigate imprint because of arresting sound waves. These simulations show good agreement with OMEGA implosion experiments. |
Thursday, November 8, 2018 4:12PM - 4:24PM |
UO4.00012: Cryogenic Target Performance and Fuel-Ablator Perturbation Growth Timothy J Collins, Christian Stoeckl, Reuben Epstein, Johan Frenje, Maria Gatu Johnson, Richard David Petrasso, Riccardo Betti, Jacques Alain Delettrez, Wade Bittle, Chad Forrest, Vladimir Glebov, Valeri N Goncharov, David R Harding, Igor Igumenshchev, Douglas Jacobs-Perkins, Roger Janezic, John Kelly, Tanya Kosc, Chad Mileham, Robert Lee McCrory, Patrick McKenty, Frederic J Marshall, Radha Bahukutumbi, Sean P Regan, Thomas C Sangster OMEGA cryogenic target implosions show a performance boundary that has been correlated with shell stability during the acceleration phase of the implosion: for sufficiently low adiabats and high in-flight aspect ratios (IFAR’s) the measured neutron-weighted shell areal density and neutron yield relative to the clean simulated yield sharply declines. This is thought to be indicative of disruption of the shell because of the Rayleigh-Taylor instability. Direct evidence of this was previously obtained using a Si Heα backlighter driven by a ~20-ps short pulse generated by OMEGA EP. The shadow cast by the shell shortly prior to stagnation, as diagnosed using backlit radiographs, shows a softening near the limb, which is evidence of an ablator-fuel mix region for low adiabat implosions (α ~ 1.9, IFAR = 14), but not for higher-adiabat implosions (α ~ 2.5, IFAR = 10). We present comparison of synthetic radiographs of 2-D simulations, investigating the effectiveness of imprint and other mechanisms (diffusive mix, interfacial surface perturbations) for generating the inferred fuel-ablator mix. |
Thursday, November 8, 2018 4:24PM - 4:36PM |
UO4.00013: Signatures of Laser Imprint in OMEGA Cryogenic Implosions Radha Bahukutumbi, Kenneth Anderson, Ricccardo Betti, Edward Campbell, Duc M Cao, Timothy J Collins, Timothy J Collins, Chad Forrest, Suxing Hu, James P Knauer, John A Marozas, Vladimir Glebov, Valeri N Goncharov, Sean P Regan, Thomas C Sangster, Rahul C Shah, A. Shvydky, Christian Stoeckl, Johan Frenje, Maria Gatu Johnson, Richard David Petrasso The growth of single-beam nonuniformity or laser imprint can compromise target performance in direct-drive implosions. With increasing in-flight aspect ratio (IFAR is the ratio of the shell radius to shell thickness), the imprint front can penetrate through the shell, reducing the in-flight shell density. This results in a less-efficient piston for assembling the hot spot. The role of laser imprint on target performance is discussed in this presentation. Simulations of OMEGA cryogenic implosions with varying IFAR are presented. These DRACO simulations include a 3-D ray trace with the effect of cross-beam energy transfer, nonlocal heat conduction, and calculated first‑principles equations of state and opacities of the ablator and ice. Observables including ablation-surface trajectories, neutron yields, areal densities, neutron rates, ion temperatures, and hot-spot images are compared to experiment. Specific signatures including broadened neutron rates, reduced areal densities, and thicker shells relative to spherically symmetric simulations are presented. |
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