Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session ZI02: ICF: Implosions, Burn and Hohlraum PhysicsInvited Live
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Chair: John Moody, Lawrence Livermore National Laboratory Room: Ballroom C |
Friday, November 12, 2021 9:30AM - 10:00AM |
ZI02.00001: Design of inertial fusion implosions reaching the burning plasma regime with the I-Raum Invited Speaker: Christopher V Young A burning plasma state [1] has recently been achieved for the first time via indirect-drive inertial confinement fusion (ICF) at the National Ignition Facility (NIF). One of the last remaining milestones in laboratory fusion research before reaching ignition, a burning plasma occurs when energy deposited from alpha particles produced in deuterium-tritium (DT) fusion reactions becomes the dominant source of heating in the plasma. Four NIF experiments recently surpassed the burning plasma threshold, two using traditional cylindrical hohlraums [3] and two using a modified cylindrical geometry with recessed pockets, dubbed the “I-Raum” [2]. Both low gas fill platforms increased hohlraum efficiency while maintaining control over implosion symmetry, enabling larger capsules to be fielded at fixed maximum laser drive energy, resulting in record performance. |
Friday, November 12, 2021 10:00AM - 10:30AM |
ZI02.00002: Assessing prospects for megajoule-class yields on the National Ignition Facility Invited Speaker: Daniel S Clark What implosion conditions will be required to reach megajoule-class yields on the National Ignition Facility? While experimental progress is being made toward the goal of ignition, for the moment, this question can only be answered theoretically or computationally. Drawing on the data base of NIF experiments to-date, this talk reviews the results of detailed, three-dimensional modeling of NIF cryogenic implosions and the results of extrapolating those simulations to the threshold of ignition. Two approaches are followed: direct hydrodynamic scaling of post-shot simulations of recent NIF implosions to megajoule yields, and an assessment of new, higher-compression designs aiming to lower the threshold for ignition. The first approach assumes no improvement in implosion quality but an upgrade to NIF's power and energy capabilities or an increase in hohlraum-to-capsule coupling efficiency. The second, complementary approach exploits learning from past experiments to envision higher compression but stable implosions with reduced ignition requirements. Of course, both approaches can also be pursued in combination. Based on this work, capsule absorbed energies greater than 400 kJ are required to reach ignition if there are no further improvements in implosion quality. However, designs reaching compressions ~20% higher than the current best performing experiments could significantly lower this energy requirement if they can be effectively imploded. |
Friday, November 12, 2021 10:30AM - 11:00AM |
ZI02.00003: Diagnosing hot-spot ρR and asymmetries in ICF implosions at the National Ignition Facility using directional secondary DT-neutron spectra Invited Speaker: Brandon J Lahmann Obtaining a fundamental understanding of the impact of fuel-ablator mix, drive asymmetries and capsule-engineering features on the performance of an Inertial Confinement Fusion (ICF) implosion is essential to the Program at the National Ignition Facility (NIF). Traditionally, x-ray techniques have been used to probe these phenomena through measurements of the hot-spot shape. A new window on hot-spot ρR and asymmetries has been opened through measurements of directional secondary DT-neutron spectra. In contrast to the x-ray image that reflects the plasma electron profile, the yield of the secondary DT neutrons reflects triton's average range in the deuterium plasma. As these techniques have different sensitivities to fuel-ablator mix, drive asymmetries and capsule-engineering features, they are very complementary. In this groundbreaking work, it is demonstrated that absolute secondary-DT neutron spectra provide accurate information on the convergence ratio (CR) that correlates with the true convergence of an implosion. It was also shown that the secondary-DT-neutron-derived CR properly captures the deleterious effects of drive asymmetries and engineering features. The CR inferred from x-ray imaging techniques was shown to respond differently to these phenomena, albeit highly correlated with the CR inferred from secondary DT-neutron technique. This is most likely an indication the ablator gets mixed into the fuel when the implosion is asymmetric causing increased brightness in the central region. In addition, it was shown that the relative widths of directional secondary-DT neutron spectra are strongly correlated with a mode-2 asymmetry of the hot spot, which is supported by multi-dimensional simulations. This work was supported in part by the US DOE, LLE, LLNL and DOE NNSA COE. |
Friday, November 12, 2021 11:00AM - 11:30AM |
ZI02.00004: Advances toward hydro-equivalent ignition in OMEGA direct-drive implosions Invited Speaker: Varchas Gopalaswamy Recent progress in direct-drive inertial confinement fusion has considerably improved the prospects for achieving thermonuclear ignition with megajoule-class lasers. recent OMEGA implosions are expected to produce about 600 kJ of fusion yield and 75% of the Lawson triple product required for ignition at 1.9MJ of symmetric illumination when hydrodynamically scaled to laser energies typical of the National Ignition Facility (NIF). Those implosions have benefited from a significant increase in implosion velocity obtained through larger-diameter targets. A new statistical approach [Gopalaswamy et al, Nature 565 (2019) 581–586.] used in designing OMEGA targets has demonstrated a considerable predictive capability, thereby enabling the design of targets with improved performance, leading to quadrupling the fusion yield and increasing the areal density by over 60%. Recent record yield of 1.75e14 was achieved with 1.02 mm size capsules. Systematic experiments such as scans of SSD (smoothing by spectral dispersion) bandwidth, age of the DT fill and beam over target size are used to identify mechanisms of performance degradation and implosion optimization. Ongoing experiments test improved coupling by zooming of the laser beams after the initial picket and the picket-only laser smoothing using the multi-driver configuration (MPD) where SSD is turned off after the picket and the laser beam diameter is reduced from 831 to 751 mm. New Si-doped CH ablators are used to suppress the two plasmon decay (TPD) instability, reduce hot electrons and improve the areal densities. All these improvements are expected to further augment implosion performance toward the goal of demonstrating implosion core conditions that scale to ignition at NIF energies. Based on these recent results, a path to demonstrating hydro-equivalent ignition on OMEGA is presented. |
Friday, November 12, 2021 11:30AM - 12:00PM |
ZI02.00005: Nonuniformity in Direct-Drive Implosions Caused by Polarization Smoothing Invited Speaker: Dana H Edgell Although direct-drive inertial confinement fusion targets on OMEGA are illuminated by 60 laser beams with the intention of producing a spherical, 1-D implosion, scattered light is observed to be very asymmetric in contrast to most predictions. In an attempt to optimize irradiation uniformity, each beam is split into two orthogonal polarizations using a distributed polarization rotator (DPR). This is meant to provide instantaneous smoothing by producing two copies of each beam’s speckle pattern that are offset 90 mm on target, effectively doubling the number of beams. While this ensures polarization diversity within the center of each beam, areas near the edge remain linearly polarized because of the far-field shift. For the first time, we have quantified the scattered-light intensity and polarization from individual beams and identified the DPR’s as the source of enhanced nonuniformity. The asymmetry is caused by the polarization dependence of cross-beam energy transfer (CBET)—a process known to strongly impact the coupling of laser energy to the implosion—since beams exchange more or less energy depending on the alignment of their polarizations in the linear edge regions. Agreement with the scattered-light data is significantly improved using a 3-D CBET code that tracks the individual polarizations of each sub-beam. The code predicts there are significant low-mode asymmetries in laser absorption that accompany the nonuniform scattered light, which are expected to degrade implosion performance.[1]. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856. [1] D. H. Edgell et al., “Nonuniform Absorption and Scattered Light in Direct-Drive Implosions Driven by Polarization Smoothing,” Physical Review Letters (accepted for publication). |
Friday, November 12, 2021 12:00PM - 12:30PM |
ZI02.00006: Precision Measurements of Hohlraum L-shell Preheating in Tungsten-based Double Shells and their Consequences for Shape and Stability Control Invited Speaker: Eric N Loomis To approach the ideal, one-dimensional thermonuclear burn performance of double shell Inertial Confinement Fusion (ICF) capsules, detrimental sources of implosion asymmetry must be controlled [D.S. Montgomery et al., Phys. Plasmas 25 (2018)]. These asymmetries span a broad spectrum from low-modes, born during early stages of the outer shell implosion, to high-mode instability growth from surface roughness [P. Amendt et al., Phys. Plasmas 10 (2003)]. Hard x-ray preheat is hypothesized to be a controlling factor in how modes across this range transfer and grow between shells. |
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