Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session UI3: ICF: Implosion DriveInvited
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Chair: Kyle Peterson, Sandia National Laboratories Room: 210 ABEF |
Thursday, November 3, 2016 2:00PM - 2:30PM |
UI3.00001: Progress towards a one-dimensional layered DT implosion using HDC capsules at the NIF Invited Speaker: Laurent Divol Using a 0.8x scale HDC capsule (D=1.6 mm) in a full scale DU hohlraum (D=5.75 mm) filled with relatively low He gas (0.3mg/cc), we have been able to achieve a high (C=26) convergence layered DT implosion that is diagnosed within 10 percent of round at all measured times. An adiabat-2.5, 3-shock, 1MJ-7ns laser pulse was used to achieved velocities $>$350 km/s, neutron yield $\approx$ 3e15 with a down scattered ratio $\approx$ 0.03. This platform shows minimal laser plasma interaction (no measurable hot electrons, $> 97\%$ coupling, no cross beam energy transfer required). A direct control of the laser cone fraction vs. time was used to obtain 3-shock-breakout symmetry (keyhole target), in flight symmetry (radiography at convergence 2-4) and symmetric hot spot/rebound shock at convergence 12 (gas-filled capsule) and 26 (layered DT). Further repointing of laser cones demonstrated control of higher modes (P4). 4 layered DT implosions allowed to compare the effect of W-dopant, symmetry and velocity on performance. We will show using experimental results and simulations that the W-doped HDC implosion behaves as expected and reaches 40$\%$ of Yield Over Clean (YOC), with the fill-tube perturbation being a possible cause of the reduced yield. The undoped HDC capsule has a YOC $<$ 0.3, showing more sensitivity to X-ray preheat than expected. The path towards an equivalent scale 1 implosion capable of large alpha-heating will be discussed. [Preview Abstract] |
Thursday, November 3, 2016 2:30PM - 3:00PM |
UI3.00002: Examining the radiation drive asymmetries present in implosion experiments at the National Ignition Facility Invited Speaker: Arthur Pak Understanding the origin, interplay, and mitigation of time dependent radiation drive asymmetries is critical to improving the performance of indirectly driven implosion experiments. Recent work has successfully modeled many aspects of the observed symmetry in implosions using the so-called high foot radiation drive [1] by applying a semi-empirical fit to the low mode time dependent flux asymmetries that the capsule experiences [2]. In these experiments, laser plasma interactions, including cross beam energy transfer, inverse Bremsstrahlung absorption, and stimulated Raman and Brillouin scattering, make controlling the symmetry of the radiation flux that drives the implosion challenging. More recently, control of implosion symmetry without the use of cross beam energy transfer, in hohlraums with lower gas fill densities using both plastic and high density carbon ablators, have been explored [3-4]. The aim of these experiments was to reduce the amount of highly non-linear laser plasma interactions and develop implosions in which the radiation flux symmetry could be more easily understood and controlled. This work describes the experimental reemission, shock timing, radiography, and x-ray self emission measurements that inform our understanding of time dependent radiation drive asymmetries. This data indicates that in the high foot series of implosion experiments, the drive asymmetry initialized during the first shock of the implosion was enhanced by the asymmetry that develops during the peak of the radiation drive. In contrast, in lower gas filled hohlraum experiments, a reduction in the magnitude of time dependent radiation asymmetries has been observed. Incorporating additional data and modeling, this work seeks to further our understanding of the physical mechanisms that currently limit symmetry control in implosion experiments.\newline [1] T. R. Dittrich, et al. ``Design of a high-foot high-adiabat icf capsule for the national ignition facility,'' Phys. Rev. Lett. \textbf{112}, 055002 (2014). [2] A. Kritcher, et al. ``Integrated modeling of cryogenic layered Highfoot experiments at the NIF'', Physics of Plasmas, \textbf{23}, 05709 (2016) [3] D. Turnbull, et al. ``Symmetry control in subscale near-vacuum hohlraums'', Physics of Plasmas, \textbf{23}, 052710 (2016) [4] D. Hinkel, et al. ``Development of improved radiation drive environment for High Foot implosions at National Ignition Facility'', submitted 2016. [Preview Abstract] |
Thursday, November 3, 2016 3:00PM - 3:30PM |
UI3.00003: Progress towards a more predictive model for hohlraum radiation drive and symmetry Invited Speaker: Ogden Jones The high flux model (HFM)\footnote{ M. D. Rosen, H. A. Scott, D. A Hinkel, et al., High Energy Density Physics \textbf{7}, 180 (2011)} was first developed to match emission levels observed\footnote{ E. L. Dewald, M. D. Rosen, S. Glenzer, et al., Phys. Plasmas \textbf{15}, 072706 (2008)} from Au spheres illuminated symmetrically at the UR-LLE OMEGA laser. It utilizes a modern non-LTE atomic physics model and an electron thermal flux limiter of 0.15 or a non-local electron transport model. Shortly thereafter, the HFM was also found to better match the radiation drive observed through the laser entrance hole of laser-heated vacuum hohlraums on the NIF.\footnote{ R. E. Olson, L. J. Suter, J. L. Kline, et al., Phys. Plasmas \textbf{19}, 053301 (2012)} Subsequent capsule implosion experiments driven by hohlraums filled with 1-1.6 mg/cc of He, having case-to-capsule ratios of $\sim$2.6, and pulse lengths $\sim$15-20 ns have been characterized by relatively large amounts of laser backscatter losses (up to 18\% of the input laser energy). They have also utilized cross beam energy transfer (CBET) to transfer power to the lasers depositing energy near the hohlraum waist. When the HFM is applied to these experiments, the hohlraum x-ray drive is over-predicted by $\sim$20-30\% during peak laser power, and the drive symmetry cannot be matched without making ad hoc corrections.\footnote{ O. S. Jones, C. J. Cerjan, M. M. Marinak, et al., Phys. Plasmas \textbf{19}, 056315 (2012)} More recent experiments using hohlraum fills from 0-0.6 mg/cc He, case-to-capsule ratios ~3-4, and pulse lengths 6-10 ns have little or no CBET or backscatter and are in better agreement with calculations using the HFM, although discrepancies remain. Uncertainties remaining in the computational models of emissivity, laser absorption, heat transport, etc. used in our hydrodynamic codes can significantly affect predictions. In this work we test various physically-plausible adjustments or alternatives to these models in order to find a more predictive model for radiation drive in the regime with little or no backscatter or CBET. We utilize measurements of the radiation drive, shape and trajectory of the imploding shell, shape of the stagnated hot spot, and bang time in capsule implosions and spectroscopic measurements of the hohlraum plasma conditions to compare against high resolution hydrodynamic calculations using the various adjusted-physics models. The goal of this work is to find a physically based model that can better predict the radiation drive and symmetry in this regime. [Preview Abstract] |
Thursday, November 3, 2016 3:30PM - 4:00PM |
UI3.00004: A Wave-Based Model for Cross-Beam Energy Transfer in Direct-Drive Inertial Confinement Fusion Implosions Invited Speaker: J.F. Myatt Cross-beam energy transfer (CBET) is thought to be responsible for an $\sim 30\% $ reduction in hydrodynamic coupling efficiency on OMEGA and up to 50{\%} at the ignition scale for direct-drive (DD) implosions. These numbers are determined by ray-based models that have been developed and integrated within the radiation--hydrodynamics codes \textit{LILAC} (1-D) and \textit{DRACO} (2-D). However, ray-based modeling of CBET in an inhomogeneous plasma assumes a steady-state plasma response, does not include the effects of beam speckle, and ray caustics are treated in an \textit{ad hoc} manner. Nevertheless, simulation results are in good qualitative agreement with implosion experiments on OMEGA (when combined with a model for nonlocal heat transport). The validity of the modeling for ignition-scale implosions has not yet been determined. To address the physics shortcomings, which have important implications for DD inertial confinement fusion, a new wave-based model has been constructed. It solves the time-enveloped Maxwell equations in three-dimensions, including polarization effects, plasma inhomogeneity, and open-boundary conditions with the ability to prescribe beams incident at arbitrary angles. Beams can be made realistic with respect to laser speckle, polarization smoothing, and laser bandwidth. This, coupled to a linearized low-frequency plasma response that does not assume a steady state, represents the most-complete model of CBET to date. New results will be presented and the implications for CBET modeling and mitigation will be described. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE{\-}NA0001944, in collaboration with J. G. Shaw, R. K. Follett, and D. H. Edgell (LLE). [Preview Abstract] |
Thursday, November 3, 2016 4:00PM - 4:30PM |
UI3.00005: Improved ICF implosion performance through precision engineering features Invited Speaker: Christopher Weber The thin membrane that holds the capsule in-place in the hohlraum is recognized as one of the most significant contributors to reduced performance in indirect drive inertial confinement fusion (ICF) experiments on the National Ignition Facility (NIF). This membrane, known as the ``tent'', seeds a perturbation that is amplified by Rayleigh-Taylor and can rupture the capsule. The ICF program is undertaking a major effort to develop a less damaging capsule support mechanism. Possible alternatives include micron-scale rods spanning the hohlraum width and supporting either the capsule or stiffening the fill-tube, a larger fill-tube to both fill and support the capsule, or a low-density foam layer that protects the capsule from the tent impact. In addition to the challenges presented by nano and microscale engineering, it is difficult to model and experimentally verify improvement from these changes. The 3D nature of the proposed replacements and the radiation shadows they cast on the capsule prohibit direct simulation. Therefore a combination of reduced models and experimental verification are used to set requirements and down-select the options. Ultimately the improved capsule support will be used to repeat a DT-layered implosion and demonstrate improved performance. [Preview Abstract] |
Thursday, November 3, 2016 4:30PM - 5:00PM |
UI3.00006: On the helical instability and efficient stagnation pressure production in thermonuclear magnetized inertial fusion Invited Speaker: A. B. Sefkow Magneto-inertial fusion experiments produce thermonuclear neutrons from plasma compressed to high convergence via z-pinch. Fusion fuel contained within a cylindrical metal liner is premagnetized with an axial field and laser-preheated prior to the liner's implosion by the JxB force. Convergence greater than 40 is inferred from x-ray self-emission spectroscopy and backlit x-ray radiography. The unprecedented stability is enabled by helical modes induced in the magnetized liner, the cause of which will be discussed, because of the suppression of the ubiquitous m=0 modes of the magneto-Rayleigh-Taylor instability found in many z-pinch implosions. The plasma temperature and flux are compressed to several keV and 100 MG at stagnation, enough to magnetically trap alpha particles and provide ``bootstrap" self-heating when scaled to larger fusion yields with DT fuel. We present quantitative comparison between experimental observables and 3D modeling in support of the interpretation that this approach to laboratory fusion can scale to larger thermonuclear yields. Namely, the implosions efficiently convert liner kinetic energy to stagnated fuel internal energy with the expected pressures of 1 Gbar and burn durations of 2 ns, in agreement with both 2D and 3D modeling. Therefore, the analysis indicates the magnetized hot-spot dynamics are not dominated by implosion instability or residual kinetic energy in our best-performing experiments, wherein laser-induced non-fuel mix into the forming hot spot is low. \newline \newline References: T. J. Awe, et al., \textit{Phys. Rev. Lett.} \textbf{111}, 235005 (2013); A. B. Sefkow, et al., \textit{Phys. Plasmas} \textbf{21}, 072711 (2014); M. R. Gomez, et al., \textit{Phys. Rev. Lett.} \textbf{113}, 155003 (2014); S. B. Hansen, et al., \textit{Phys. Plasmas} \textbf{22}, 056313 (2015); T. J. Awe, et al., \textit{Phys. Rev. Lett.} \textbf{116}, 065001 (2016). [Preview Abstract] |
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