Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session VI01: Invited: ICF Capsule Impacts and MFELive
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Chair: Brian MacGowan, LLNL |
Thursday, November 12, 2020 2:00PM - 2:30PM Live |
VI01.00001: Impact of localized radiative loss in inertial confinement fusion implosions Invited Speaker: Arthur Pak Enhanced radiative loss from impurities that mix into the reacting deuterium tritium (DT) plasma has been identified as one of the principle degradation mechanisms that can limit fusion energy production in inertial confinement fusion (ICF) experiments [1-2]. In this work, the impact to fusion energy production due to the radiative loss from a localized mix in ICF implosions using high density carbon capsule targets is quantified [3]. The radiative loss from localized mix and local cooling of the reacting plasma conditions was quantified using multi-axis neutron and x-ray images to reconstruct the hot spot conditions during thermonuclear burn. Such localized features arise from ablator material that is injected into the hot spot from the growth of capsule surface perturbations, particularly the tube used to fill the capsule with deuterium and tritium fuel. Observations, consistent with analytic estimates, show the degradation to fusion energy production to be linearly proportional to the fraction of the total emission that is associated with injected ablator material and that this radiative loss has been the primary source of variations, of up to 1.6 times, in observed fusion energy production. Reducing the fill tube diameter has increased the ignition metric $\chi $no $\alpha $ from 0.49 to 0.72, 92{\%} of that required to achieve a burning hot spot. [1] T. Ma et al., Phys. Rev. Lett. 111, 085004 (2013). [2] S. P. Regan et al., Phys. Rev. Lett. 111, 045001 (2013). [3] A. Pak, et al. Phys. Rev. Lett 123, 1450001 (2020). [Preview Abstract] |
Thursday, November 12, 2020 2:30PM - 3:00PM Live |
VI01.00002: Experiments and simulations to understand the impact of engineering features on Inertial Confinement Fusion implosions Invited Speaker: Brian Haines Engineering features play a crucial role in determining the performance of Inertial Confinement Fusion implosions by seeding jets of cold contaminant into the hot spot[1]. Fill tubes are a dominant degradation mechanism in layered implosions on the NIF[2] and the target mount plays an important role in direct drive implosions[3]. Minimizing the impact of capsule joints and fill tubes is also critical for double shell capsules to be a viable path to ignition[4]. Nevertheless, much of our understanding of features comes from simulation and accurate prediction of their effects requires expensive 3D simulations[1,5]. We have applied the hydrogrowth radiography (HGR) platform[6] to understand the impact of the joint in double shell implosions and are designing experiments to evaluate the fill tube impact. Mitigation strategies[4] are predicted to nearly eliminate the impact of both features on double shell implosions. HGR experiments are used to reduce modeling uncertainties and validate simulations of features using xRAGE[7,8], an Eulerian radiation-hydrodynamics code, and play a key role in ensuring the viability of mitigation strategies. xRAGE is ideal for modeling the impacts of features due to its ability to easily and accurately model features with high resolution in routine simulations with adaptive mesh refinement. We will discuss our mitigation strategies, HGR experimental results, and efforts to improve and validate our modeling capabilities. [1]Haines et al., Nature Comm. 11:544, 2020. [2]Pak et al., Phys.~Rev.~Lett. 124:145001, 2020. [3]Gatu Johnson et al., Phys.~Plasmas 27:032704, 2020. [4]Haines et al., Phys.~Plasmas 26:102705, 2019. [5]Haines et al., Phys.~Plasmas 26:012707, 2019. [6]Smalyuk et al., Phys.~Rev.~Lett. 112:185003, 2014. [7]Gittings et al., Comput.~Sci.~Discov. 1:015005 2008. [8]Haines et al., Phys.~Plasmas 24:052701, 2017. [Preview Abstract] |
Thursday, November 12, 2020 3:00PM - 3:30PM Live |
VI01.00003: The impact of low-mode areal-density non-uniformities in indirect-drive implosions at the National Ignition Facility Invited Speaker: Daniel Casey To achieve hotspot ignition, inertial confinement fusion (ICF) implosions must achieve high hotspot pressures that are inertially confined by a dense shell of DT fuel. This requires high inflight shell velocity and high areal-density at stagnation. The size of the driver and scale of capsule required can be minimized by keeping a high efficiency of energy coupling from the imploding shell to the hotspot. However, significant 3D low mode asymmetries are commonly observed in indirect-drive implosions limiting the coupling of shell kinetic energy to the hotspot. To better quantify shell density asymmetry magnitudes and impacts, we have developed new analysis techniques [Casey et al., RSI 87, 11E715 (2016)] and analytic models [Hurricane et. al., POP 2020] that have been cross compared to data and simulation and recently extended beyond mode-1. We developed an analytic neutron transport model to cross-compare two independent measurements of asymmetry. They are in good agreement and show that the level of these asymmetries is significant. Recent analysis [MacGowan et al., HEDP 2020] has shown that there are multiple causes for the observed asymmetries, namely percent-level beam-to-beam laser power fluctuations and hohlraum diagnostic window losses. We will present evidence that non-uniformity in the ablator shell thickness in high-density carbon (HDC) experiments is also a significant cause for observed 3D implosion asymmetries. We will also discuss planned extensions of analysis techniques to mode 2-4 asymmetries using fluence compensated down-scattered neutron images and arrays of neutron activation detectors. [Preview Abstract] |
Thursday, November 12, 2020 3:30PM - 4:00PM Live |
VI01.00004: Bayesian Inference of Energy Transfer in Gigabar Convergent Experiments Invited Speaker: John Ruby High-energy-density experiments (HED) in convergent geometry can produce extreme thermodynamic states at pressures in excess of 1 Gbar (\textasciitilde 10$^{\mathrm{9}}$ atm). Understanding the flow of energy in these dynamic experiments is critical to constraining the states that are produced and in using these platforms to understand fundamental physics in extreme environments. Diagnostic measurements of these implosions are highly integrated (i.e., over space, time, spectrum), and quantitative deduction requires advanced approaches to how experimental data is analyzed. In this work the self-emission of an in-flight, laser-driven, glass shell interacting with a gigabar rebounding shock wave is measured giving the outgoing trajectory of the shell. These measurements in conjunction with a mechanical model of the shell, conservation laws, and Bayesian inference techniques are used to infer the complete trajectory of the shell and the temporal history of the pressure profile at the shell-fuel interface. This combination of in-situ measurements results in the temporal history of the kinetic energy of the inflight shell and the time history of the work done on the fuel by the shell in a self-consistent picture. This technique can be applied to a variety of different implosion types to infer the flow of energy and constrain the assembled HED states. [Preview Abstract] |
Thursday, November 12, 2020 4:00PM - 4:30PM Live |
VI01.00005: Understanding energy confinement in Wendelstein 7-X Invited Speaker: Per Helander The W7-X stellarator has been optimized for low neoclassical transport and has achieved confinement times exceeding 200 ms at high densities and temperatures well above 3 keV in pellet-fuelled plasmas. Neoclassical transport calculations indicate that similar density and temperature profiles could not have been attained in less optimized magnetic configurations of W7-X or other stellarators scaled to the same volume and magnetic field strength, since the neoclassical energy flux would then have exceeded the total heating power. The magnetic geometry also serves to limit the turbulent transport. Unlike in tokamaks, trapped particles need not be localized to regions of bad curvature, which reduces the drive for trapped-electron modes. Ion- and electron-temperature-gradient-driven (ITG/ETG) modes remain, but can be stabilized by the density gradient that results from pellet injection. This theoretical expectation is a likely explanation for the improved confinement in plasmas with pellets, and is supported by gyrokinetic simulations. An increased radial electric field may also help to suppress turbulence under these circumstances. In electron-cyclotron-resonance-heated plasmas without pellets, turbulent transport is strong enough to limit the ion temperature to values below 2 keV. Recent gyrokinetic simulations suggest that this limitation is due to strong ITG turbulence when the electron-to-ion temperature ratio becomes large. With the addition of more heating power, higher-density operation is likely to lead to higher ion temperatures. The fact that turbulent transport can be reduced by tailoring the magnetic-field geometry in ways that are understood theoretically is encouraging and suggests that additional optimization could further improve confinement in stellarators. [Preview Abstract] |
Thursday, November 12, 2020 4:30PM - 5:00PM Live |
VI01.00006: The physics basis to integrate an MHD stable, high-power core to a cool divertor for steady-state reactor operation Invited Speaker: Francesca Turco Coupling a high-performance core to a low heat flux to the divertor is a crucial step for ITER and any future reactor. DIII-D recently expanded the steady-state hybrid scenario to high density and divertor impurity injection, to study the impact of increased density at high power and the feasibility of a radiating mantle solution. This work presents the physics basis for the trade-offs between density, current drive and stability to tearing modes at high beta. EC power is crucial to tailor the plasma profiles into a passively stable state, and to eject impurities from the core. Off-axis EC depositions decrease the heating efficiency, but calculated electron heat transport coefficients show that this effect is partially mitigated by improved confinement inside the EC deposition. The reduction in pressure is recovered by increasing the density. This favorable scaling of confinement with density was observed in high power plasmas for years, and this work provides a comprehensive explanation. ELITE predictions indicate that a path in peeling-ballooning stability opens up for certain conditions of density, power, q95 and shaping, allowing the edge pressure to continue increasing without encountering a limit. In the core, calculated anomalous fast-ion diffusion coefficients are consistent with density fluctuation measurements in the TAE range, showing that smaller fast-ion losses contribute to the enhanced confinement at high density. The edge integration study shows that divertor heat loads can be reduced with Ne and Ar injection, but this eventually triggers a cascade of n$=$1,2,3 core tearing modes. We can now show that impurity radiation in the core is small and it is not the cause for the drop in confinement at high Ar and Ne injection. The overlap between the core tearing modes is consistent with the loss of pressure as estimated by the Belt model for the coupled rational surfaces. Optimization of these trade-offs has achieved plasmas with sustained H98y2$=$1.7, fGW$=$0.7 and $\sim $85{\%} mantle radiation. [Preview Abstract] |
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