Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session BI2: Ablators, Instabilities, and Asymmetries |
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Chair: Russell Follett, University of Rochester Room: 102ABC |
Monday, October 23, 2017 9:30AM - 10:00AM |
BI2.00001: Exploring the limits of case-to-capsule ratio, pulse length, and picket energy for symmetric hohlraum drive on NIF Invited Speaker: Debra Callahan Over the past two years, we have been exploring low gasfill hohlraums (He fill at 0.3-0.6 mg/cc) as an alternate to the high gasfill hohlraums used in NIC and the High Foot campaigns (He fill at 1-1.6 mg/cc). These low fill hohlraums have significantly reduced laser-plasma instabilities and increased coupling to the target as compared to the high fill hohlraums and take us to a new region of parameter space where the hohlraum is limited by hydrodynamic motion of the hohlraum wall rather than by laser plasma interactions. The outer cone laser beams interacting with the hohlraum wall produce a ``bubble'' of low density, high Z material that moves toward the center of the hohlraum. This gold or depleted uranium bubble eventually intercepts the inner cone beams and prevents the inner cone beams from reaching the waist of the hohlraum – where they are needed to get a symmetric implosion. Thus, the speed of the bubble expansion sets the allowable pulse duration in a given size hohlraum. Data and simulations suggest that the bubble is launched by the early part of the laser pulse (“picket”) and the gold/gas interfaces moves nearly linearly in time toward the axis of the hohlraum. The velocity of the bubble is related to the square root of the energy in the picket of the pulse – thus the picket energy and pulse duration set the allowable hohlraum size and case-to-capsule ratio. In this talk, will discuss a data based model to describe the bubble motion and apply this model to a broad set of data from a variety of ablators (CH, HDC, Be), pulse durations (6-14 ns), case-to-capsule ratios (rhohl/rcap of 3-4.2), hohlraum sizes (5.4-6.7 mm diameter), and hohlraum gasfill densities (0.3-0.6 mg/cc). We will discuss how this model can help guide future designs and how improvements in the hohlraum (foam liners, hohlraum shape) can open up new parts of parameter space. [Preview Abstract] |
Monday, October 23, 2017 10:00AM - 10:30AM |
BI2.00002: Comparison of plastic, high-density carbon, and beryllium as NIF ablators Invited Speaker: Andrea Kritcher An effort is underway to compare the three principal ablators for National Ignition Facility (NIF) implosions: plastic (CH), High Density Carbon (HDC), and beryllium (Be). This presentation will summarize the comparison and discuss in more detail the issues pertaining to hohlraum performance and symmetry. Several aspects of the hohlraum design are affected by the ablator properties, as the ablator constrains the first shock and determines the overall pulse length. HDC targets can utilize shorter pulse lengths due to the thinner, higher density shell, and should be less susceptible to late time wall motion. However, HDC requires a larger picket energy to ensure adequate melt, leading to increased late time wall movement. Be is intermediate to CH and HDC in both these regards, and has more ablated material in the hohlraum. These tradeoffs as well as other design choices for currently fielded campaigns are assessed in this work. To assess consistently the radiation drive and symmetry, integrated postshot simulations of the hohlraum and capsule were done for each design using the same methodology. The simulation results are compared to experimental data. Using this post-shot model, we make a projection of the relative plausible performance that can be achieved, while maintaining adequate symmetry, using the full NIF laser, i.e. 1.8 MJ/500 TW Full NIF Equivalent (FNE). The hydrodynamic stability of the different ablators is also an important consideration and will be presented for the current platforms and projection to FNE. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, October 23, 2017 10:30AM - 11:00AM |
BI2.00003: High-Energy-Density--Physics Studies for Inertial Confinement Fusion Applications* Invited Speaker: S.X. Hu Accurate knowledge of the static, transport, and optical properties of high-energy-density (HED) plasmas is essential for reliably designing and understanding inertial confinement fusion (ICF) implosions. In the warm-dense-matter regime routinely accessed by low-adiabat ICF implosions,\footnote{ S. X. Hu \textit{et al.}, Phys. Rev. Lett. \textbf{104,} 235003 (2010); \textit{ibid.} Phys. Rev. B \textbf{84}, 224109 (2011).} many-body strong-coupling and quantum electron degeneracy effects play an important role in determining plasma properties. The past several years have witnessed intense efforts to assess the importance of the microphysics of ICF targets, both theoretically and experimentally. On the theory side, first-principles methods based on quantum mechanics have been applied to investigate the properties of warm, dense plasmas. Specifically, self-consistent investigations have recently been performed on the equation of state, thermal conductivity, and opacity of a variety of ICF ablators such as polystyrene (CH), beryllium, carbon, and silicon over a wide range of densities and temperatures.\footnote{ S. X. Hu, T. R. Boehly, and L. A. Collins, Phys. Rev. E \textbf{89,} 063104 (2014); $^{\mathrm{\thinspace }}$S. X. Hu \textit{et al.}, \textit{ibid}. \textbf{92}, 043104 (2015).\par $^{\mathrm{3\thinspace }}$S. X. Hu \textit{et al.}, Phys. Plasmas \textbf{23,} 042704 (2016).\par $^{\mathrm{4\thinspace }}$S. X. Hu \textit{et al.}, Phys. Rev. B \textbf{94,} 094109 (2016); S. X. Hu \textit{et al.}, Phys. Rev. E \textbf{95,} 043210 (2017).\par $^{\mathrm{5\thinspace }}$S. X. Hu, ``Continuum Lowering and Fermi-Surface Rising in Strongly Coupled and Degenerate Plasmas,'' to be submitted to Physical Review Letters.}$^{\mathrm{-5}}$ In this talk, we will focus on the most-recent progress on these \textit{ab initio} HED physics studies, which generally result in favorable comparisons with experiments. Upon incorporation into hydrocodes for ICF simulations, these first-principles ablator-plasma properties have produced significant differences over traditional models in predicting 1-D target performance of ICF implosions on OMEGA and direct-drive--ignition designs for the National Ignition Facility. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. *In collaboration with L. A. Collins, T. R. Boehly, G. W. Collins, J. D. Kress, and V. N. Goncharov. [Preview Abstract] |
Monday, October 23, 2017 11:00AM - 11:30AM |
BI2.00004: Probing the seeding of hydrodynamic instabilities from non-uniformities in ablator materials using 2D velocimetry Invited Speaker: Suzanne Ali Despite the extensive work done to characterize and improve the smoothness of ablator materials used in inertial confinement fusion, features indicative of seeded instabilities from these materials are still observed. A two-dimensional imaging velocimetry technique has been used on Omega (OHRV 2D-VISAR system) to measure the velocity roughness of shock fronts launched by indirect drive in the three ablator materials of current interest. We have used this diagnostic, coupled with extensive pre-shot target metrology, to study the presence of shock-front perturbations in GDP, beryllium, and high density carbon ablators. Observed features are small variations from one-dimensional evolution, but are important for fully understanding the effects of surface topography, dynamic material response, and internal heterogeneities on the stability of ICF capsules. For all three ablators we have quantified perturbations that can dominate conventional surface roughness seeds to hydrodynamic instability. [Preview Abstract] |
Monday, October 23, 2017 11:30AM - 12:00PM |
BI2.00005: Measurement and mitigation of X-ray shadow imprint of hydrodynamic instabilities on the surface of Inertial Confinement Fusion capsules due to the fill tube Invited Speaker: Andrew MacPhee Indirectly-driven Inertial Confinement Fusion (ICF) implosions on the National Ignition Facility (NIF) employ a small diameter (10$\mu $m) fill tube to supply the cryogenic deuterium-tritium (DT) fuel to the capsule. Recent experimental observations characterizing the perturbation produced by this fill tube have revealed an unexpected shadow imprinted instability mechanism [1], whereby several of the x-ray spots formed on the inside wall of the hohlraum cast directional shadows of the fill tube onto the surface of the capsule. Reduced ablation in the corresponding umbrae of these shadows leads to a pattern of radial ridges of excess ablator material measuring \textasciitilde 100 nm above the surrounding capsule surface. By the time the capsule has converged \textasciitilde 2x from its original radius, the areal density ($\rho $R) perturbation of these spoke-like features becomes comparable to that of central hole due to the fill tube itself. We report both quantitative radiographic measurements of this newly observed perturbation (for several ablator materials) as well as the results of two strategies for mitigating against such shadow imprinted instabilities: 1.) reducing the fill tube diameter and wall thickness to produce a smaller perturbation that blows down to low density more quickly, and 2.) modifying the driving laser pulse for the lower-intensity inner beams to allow more time for the fill tube to blow down to low density prior to the onset of shadow imprint, which is produced by the more-intense outer beams during the later part of the drive. Results and analysis from both focused radiographic experiments as well as the impact on the performance of layered DT ignition implosions will be discussed. [1] A. G. MacPhee et al., Phys. Rev. E 95, 031204(R) (2017) *Work performed under the auspices of the U.S. D.O.E. by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
Monday, October 23, 2017 12:00PM - 12:30PM |
BI2.00006: A near one-dimensional 2-shock indirectly driven implosion at convergence ratio \textasciitilde 30 Invited Speaker: Steve MacLaren Inertial confinement fusion implosions at the National Ignition Facility, while successfully demonstrating self-heating due to alpha-particle deposition, have fallen short of the performance predicted by one-dimensional multi-physics implosion simulations.~ The current understanding, based on simulations as well as experimental evidence, suggests that the principle reason for the disagreement is a breeching of the cold fuel assembly at stagnation which would otherwise completely confine the hot spot.~ 3-D simulations indicate a combination of low-mode symmetry swings and ablation-front hydrodynamic instability seeded by engineering features such as the capsule tent and fill tube lead to localized thinning and perforation of the stagnated fuel, resulting in a loss of hot spot pressure and energy.~ We describe a short series of experiments on the NIF designed specifically to avoid these issues in order to understand if, once they are removed, a suspended-fuel-layer deuterium-tritium implosion can achieve 1-D simulated performance.~ The particular implosion system combines a thick capsule shell with an elevated initial ablation temperature to minimize the ablation front perturbations from the engineering features, and incorporates a large ratio of hohlraum-to-capsule radius as a means to permit a higher degree of control over implosion symmetry.~ The resulting implosion at a convergence ratio of \textasciitilde 30 was not perfectly spherically symmetric as observed by both neutron and time-resolved x-ray imaging diagnostics.~ However, the stagnation observables match closely the performance predicted by 1D simulations, including, when some hot spot motion is accounted for, the apparent ion temperature. We present this result along with the design for an upcoming 2-shock experiment to test whether this level of agreement with the 1D model can be achieved in the self-heating regime. This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344 [Preview Abstract] |
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