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 NI2: Inertial Confinement Fusion |
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Chair: Maria Gatu-Johnson, Massachusetts Institute of Technology-MIT Room: OCC Oregon Ballroom 203 |
Wednesday, November 7, 2018 9:30AM - 10:00AM |
NI2.00001: Approaching a burning plasma on the NIF Invited Speaker: Omar A Hurricane After the National Ignition Campaign, our program to achieve ignition initiated a more exploratory approach starting from less stressful, lower convergence implosions. By probing away from a more conservative implosion in-steps towards conditions of higher velocity and compression, Fuel Gain and alpha-heating were obtained. In the process, performance cliffs were identified the most damaging of which were symmetry control of the shell of the implosion and adverse hydro seeded by engineering features that penetrated the shell. From 2015-2017 we focused on reducing LPI by moving to larger hohlraums with lower gas fill densities to improve symmetry control and on reducing the impact of the fill-tubes and capsule mounting. A broad parameter space was explored [1] including capsule material and size, laser pulse length, hohlraum size and gas fill and implosion velocity and compression. The results were much more efficient implosions that obtained the same performance levels, but with less laser energy. Presently, we have several implosions [2,3] poised to step into a burning plasma state where alpha particle heating is the dominant source of heating. The data from this exploration supported by focused experiments shed light on key target physics and has provided a data-based road-map to optimize the energy coupled to the capsule and subsequently to the hot spot – our HYBRID (high-yield big radius implosion design) strategy. Here we describe the key data underpinning our principal conclusions and hypotheses and the foundation of this data-based approach to increasing the energy coupled to the hot spot in an attempt to take the next step to a burning plasma and ultimately ignition. [1] Callahan, D. et. al., PoP, 25, 056305 (2018) [2] Casey, D.T, et al., PoP, 25, 056308 (2018) [3] Le Pape, S., et al., PRL, 120, 045003 (2018)
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Wednesday, November 7, 2018 10:00AM - 10:30AM |
NI2.00002: Multi-objective data analysis of performance-degradation trends in ICF implosions, and implications for hydro-scaling to ignition conditions Invited Speaker: Arijit Bose Multi-objective data analysis is of increasing interest to the ICF community since the multi-faceted causes of implosion-performance degradation are not yet fully identified. This type of technique for analyzing data from cryogenic DT implosions takes simultaneously into account trends in all experimental observables, therefore, providing leads for investigating the cause of the performance degradation and systematic errors in the measurements. This work is based on a concept-driven analysis of degradation trends in the implosion observables, arising from low- (ℓ<6) and mid-mode (6<ℓ<40) asymmetries, and 1-D degradations. The analysis technique was first applied to an ensemble of cryogenic DT implosions that generated hot-spot pressures of ~50 Gbar. The 1-D physics models had been previously adjusted for these implosions. All experimental observables pertaining to the core could be reconstructed with the same combination of low and mid modes, suggesting a systematic and repeatable mechanism causing the performance degradation. In addition to low modes, which cause degradation of the stagnation pressure, it is found that mid modes are present and that they reduce the size of the neutron and x-ray producing volumes, and augment the measured areal density. The mid modes are most likely introduced by laser drive non-uniformity caused by the superposition of the 60 laser beams on OMEGA. A discrepancy between measured and simulated burn width and bang time was also observed, suggesting a systematic error in the measurements. Correcting for these low- or mid-mode asymmetries would increase the hot-spot pressure from 56 Gbar to ~80 Gbar at OMEGA, and generate a burning plasma when hydro-scaled to 2 MJ direct illumination. Recently, the technique was used to analyze the low converging direct-drive implosions which brought to light general inconstancies in the 1-D physics models. |
Wednesday, November 7, 2018 10:30AM - 11:00AM |
NI2.00003: Magnetic Fields in Indirect-drive Inertial Confinement Fusion Invited Speaker: Christopher Alexander Walsh Plasma magnetisation effects are routinely ignored in the design and interpretation of laser-driven inertial confinement fusion (ICF) experiments. The presented work uses the 3-D extended-magneto-hydrodynamics code Chimera to simulate both the impact of self-generated magnetic fields on the hotspot cooling process and assess the anticipated increase in fusion performance through the application of external magnetic fields. Magnetic fields are spontaneously generated during ICF implosions by the Biermann battery process when the capsule is not spherically symmetric. During stagnation, the hotspot edge contains large magnetic field intensities, estimated to be up to 10,000T in strength [1]. Subsequent magnetisation of the electron population can reduce thermal conductivities by 90%. Further extended-MHD processes, such as the Nernst term advecting fields out of the hotspot, result in the estimated change in yield due to self-generated fields being small. Righi-Leduc heat-flow also significantly modifies the hotspot cooling process and acts to increase the hot-spot deformation. Externally-applied magnetic fields can be used to enhance the fusion yield by magnetising both the electron and α-particle populations in the hotspot, increasing the energy containment. Modifications to hotspot shape are explored by using perturbations relevant to the high-foot (radiation asymmetry and tent scar) and HDC (fill-tube) campaigns. High-mode perturbations are also simulated, suggesting reductions of mix into the hotspot due to magnetic tension suppressing vortices. Extended-MHD effects are also important in this regime, with the cross-gradient-Nernst term twisting the magnetic field and enhancing energy containment. Finally, the overall increase in yield by applying magnetic fields to current NIF implosions will be estimated. [1] C. A. Walsh et al. Physical Review Letters 118, 155001 (2017) |
Wednesday, November 7, 2018 11:00AM - 11:30AM |
NI2.00004: Experimental study of energy and shape transfer in double shell implosions Invited Speaker: Elizabeth Merritt As high-velocity, high-convergence ratio ICF experiments continue to push for higher DT fuel burn-up fraction and -particle heating at the NIF, researchers continue to explore additional methods to study burning fusion plasmas. Recently, advances in target fabrication have made double shell (DS) implosions a viable burn platform. Core to the DS capsule is a high-Z (e.g., Au) metal pusher that accesses the “volume”-burn regime by trapping radiation losses and compressing a uniform fuel volume at reduced velocities. The DS system relies on a series of energy transfer processes starting from x-ray absorption by the outer shell, followed by collisional transfer of kinetic energy to an inner shell, and final conversion to fuel internal energy. Beyond efficient energy transfer, we must also design double shells for robust performance against engineering features and implosion asymmetry. We present simulation and experiment results on momentum and shape transfer between the outer and inner shell. We examine 1D energy transfer between shell layers using trajectory measurements from a series of surrogate targets; the series builds to a complete double shell layer by layer, isolating the physics of each step of the energy transfer process. Closely matching theory and RAGE simulations, experiments show a reduction in ablator velocity with the addition of a foam cushion, and a reduction in ablator/foam kinetic energy due to collision with the inner shell. We present ablator-only experiments demonstrating control of outer-shell shape, since simulations suggest that ablator shape during the collision primarily determines the fuel shape at stagnation. We also present results of shape transfer studies using our surrogate “imaging”-shell platform. Included in these studies is an examination of the role of the ablator joint engineering feature on implosion shape. |
Wednesday, November 7, 2018 11:30AM - 12:00PM |
NI2.00005: Suppressing Parametric Instabilities with Laser-Frequency Detuning and Bandwidth Invited Speaker: Russell Follett High-frequency laser–plasma instabilities, such as stimulated Raman scattering (SRS) and two-plasmon decay (TPD), present a major challenge for laser-driven inertial confinement fusion (ICF). The plasma waves driven in these instabilities inhibit the compression of the fusion capsule, and ultimately the gain, by scattering light and accelerating hot electrons that preheat the fuel. Attempts at mitigation have primarily focused on modifying the implosion design to limit the instability growth rate.[1] This approach severely restricts the ICF design space. The introduction of temporal incoherence in the drive lasers offers a path toward mitigation without sacrificing hydrodynamic efficiency in the implosion design. Through the use of 3-D laser–plasma interaction simulations, we show that two forms of temporal incoherence—frequency detuning and laser bandwidth—can suppress SRS and TPD. The simulations were conducted with LPSE (laser‑plasma simulation environment), which provides a unique capability to model the dynamic evolution of parametric instabilities at the long scale lengths and with the complex beam geometries encountered in ignition plasmas. The viability of each form of temporal incoherence, both in terms of instability suppression and practical implementation, is considered. For instance, a three-wavelength scheme, with the detuning available on current laser architectures, has been found to nearly eliminate TPD and the associated hot electrons.[2] A next-generation ICF driver capable of suppressing TPD and SRS with temporal incoherence will allow for higher laser intensities and ablation pressures, greatly expanding the ICF design space. [1] R. K. Follett et al., Phys. Rev. Lett. 116, 155002 (2016). [2] R. K. Follett et al., Phys. Rev. Lett. 120, 135005 (2018). |
Wednesday, November 7, 2018 12:00PM - 12:30PM |
NI2.00006: Mitigation of stimulated Brillouin scattering in NIF experiments Invited Speaker: Brian James MacGowan Stimulated Brillouin scattering (SBS) is a laser-plasma instability (LPI) that can lead to high levels of back-scattered laser energy in inertial confinement fusion (ICF) or high-energy density physics experiments on laser facilities like the National Ignition Facility (NIF). In addition to its impact on laser-target coupling and drive symmetry, SBS can also constrain experiments on NIF due to its potential to damage laser transport mirrors as the scattered light travels back through the laser chain. Recent indirect-drive ICF designs have experienced a resurgence of SBS on the outer cones of NIF beams. This is attributed to the lower gas-fill used in these hohlraums, that allows the formation of a large volume ("bubble") of gold filling up the hohlraum that is prone to SBS growth. The experiment designs also have less reliance on crossed-beam energy transfer (CBET) to tune the implosion symmetry by transferring power to the inner cones, leading to increased power on the outer cones beams. Risk of optics damage due to SBS restricts the design space of most of these recent platforms and must be considered for operation of NIF at higher power and energy. In this talk, we will review our recent efforts to understand and mitigate SBS and optics damage. Mitigation strategies have been developed and tested in simulations using the code pF3D coupled to crossed-beam energy transfer calculations; these include laser-based LPI control (higher laser bandwidth, multi-wavelength upgrade, or optimized phase plate design), target optimization (e.g. changing the hohlraum wall composition to increase ion wave damping and reduce SBS), and facility protection against SBS scattered light. We will present the estimated impact of these mitigations on current ICF platform performance, and on scaled designs at higher laser energy. |
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