Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session TI2: ICF and Z-pinch Physics |
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Chair: Patrick McKenty, University of Rochester Room: Bissonet |
Thursday, October 30, 2014 9:30AM - 10:00AM |
TI2.00001: Rosenbluth Award: First observations of Rayleigh-Taylor-induced magnetic fields in laser-produced plasmas using x rays and protons Invited Speaker: Mario Manuel Recent experiments [Manuel, PRL 108 (2012)] demonstrated the existence of self-generated B-fields from the Rayleigh-Taylor (RT) instability in laser-produced plasmas, as originally predicted by Mima et al. [Mima PRL 41 (1978)]. Misaligned density and temperature gradients caused by RT growth in ablatively driven targets generate B-fields in the plasma through the Biermann battery source. X-ray and proton radiography diagnosed areal-density and B-field perturbations in laser-irradiated targets with seeded sinusoidal surface perturbations. Inferred B-field strengths indicated ratios of thermal to magnetic pressures ($\beta )$ near the ablation surface of 10$^{4}$--10$^{5}$, suggesting no magnetic effects on ablative RT during the linear growth phase. However, the magnitude of this self-generated field increases with the perturbation height [Srinivasan, PRL 108 (2012)] and can affect morphology in the nonlinear regime. The detailed structure of highly nonlinear RT spikes is important to understand the inner wall expansion of hohlraums in indirect-drive inertial fusion and in multiple astrophysical systems, including the explosion phase of core-collapse supernovae and formation of planetary nebulae. Numerical calculations investigating the magnetic effects on nonlinear RT-spike evolution under conditions similar to previous measurements will be covered and implications discussed. \\[4pt] Support for this work was provided by NASA through Einstein Postdoctoral Fellowship grant number PF3-140111 awarded by the Chandra X-ray Center, which is operated by the Astrophysical Observatory for NASA under contract NAS8-03060. This work is funded by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasma under grant number DE-NA0001840. Previous work described here was supported in part by NLUF (DE-NA0000877), FSC/UR (415023-G), DoE (DE-FG52-09NA29553), LLE (414090-G), and LLNL (B580243). [Preview Abstract] |
Thursday, October 30, 2014 10:00AM - 10:30AM |
TI2.00002: Kinetic Effects in Inertial Confinement Fusion Invited Speaker: Grigory Kagan Sharp background gradients, inevitably introduced during ICF implosion, are likely responsible for the discrepancy between the predictions of the standard single-fluid rad-hydro codes and the experimental observations. On the one hand, these gradients drive the inter-ion-species transport, so the fuel composition no longer remains constant, unlike what the single-fluid codes assume. On the other hand, once the background scale is comparable to the mean free path, a fluid description becomes invalid. This point takes on special significance in plasmas, where the particle's mean free path scales with the square of this particle's energy. The distribution function of energetic ions may therefore be far from Maxwellian, even if thermal ions are nearly equilibrated. Ironically, it is these energetic, or tail, ions that are supposed to fuse at the onset of ignition. A combination of studies has been conducted to clarify the role of such kinetic effects on ICF performance. First, transport formalism applicable to multi-component plasmas has been developed. In particular, a novel ``electro-diffusion'' mechanism of the ion species separation has been shown to exist. Equally important, in drastic contrast to the classical case of the neutral gas mixture, thermo-diffusion is predicted to be comparable to, or even much larger than, baro-diffusion. By employing the effective potential theory this formalism has then been generalized to the case of a moderately coupled plasma with multiple ion species, making it applicable to the problem of mix at the shell/fuel interface in ICF implosion. Second, distribution function for the energetic ions has been found from first principles and the fusion reactivity reduction has been calculated for hot-spot relevant conditions. A technique for approximate evaluation of the distribution function has been identified. This finding suggests a path to effectively introducing the tail modification effects into mainline rad-hydro codes, while being in good agreement with the first principle based solution. [Preview Abstract] |
Thursday, October 30, 2014 10:30AM - 11:00AM |
TI2.00003: Impact of First-Principles Property Calculations of Warm-Dense Deuterium/Tritium on Inertial Confinement Fusion Target Designs Invited Speaker: S.X. Hu Accurate knowledge of the properties of warm dense deuterium/tritium (DT) is essential to reliably design 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).} strong-coupling and degeneracy effects play an important role in determining plasma properties. Using first-principles methods of both path-integral Monte Carlo and quantum molecular-dynamics (QMD), we have performed systematic investigation of the equation of state,\footnote{S. X. Hu \textit{et al.}, Phys. Rev. B \textbf{84}, 224109 (2011). } thermal conductivity,\footnote{V. Recoules \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 075002 (2009). } \footnote{F. Lambert \textit{et al.}, Phys. Plasmas \textbf{18}, 056306 (2011). } \footnote{S. X. Hu \textit{et al.}, Phys. Rev. E \textbf{89}, 043105 (2014). } and opacity\footnote{S. X. Hu \textit{et al.,} ``First-Principles Opacity Table of Warm-Dense Deuterium for ICF Applications,'' submitted to Physical Review E. } for DT over a wide range of densities and temperatures. These first-principles properties have been incorporated into our hydrocodes. When compared to hydro simulations using standard plasma models, significant differences in 1-D target performance have been identified for simulations of DT implosions. For low-adiabat $\left( {\alpha \le 2} \right)$ DT plasma conditions, the QMD-predicted opacities are $10$ to $100 \times$ higher than predicted by the cold-opacity--patched astrophysical opacity table. The thermal conductivity could be $3$ to $10 \times$ larger than the Lee--More model prediction. These enhancements can modify the shell adiabat and shock dynamics in lower-$\alpha $ ICF implosions, which could lead to $\sim 40\% $ variations in peak density and neutron yield. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Thursday, October 30, 2014 11:00AM - 11:30AM |
TI2.00004: Reaction-in-Flight Neutrons and the Stopping Power in Cryogenic NIF Capsules Invited Speaker: Anna Hayes Recent experiments in cryogenic DT capsules at the National Ignition Facility (NIF) observed high-energy reaction-in-flight (RIF) neutrons via threshold (\textgreater 15 MeV) neutron reactions on thulium foils. This represents the first measurements of RIF neutrons in inertial confinement fusion plasmas. RIF neutrons are produced by a two-step process. In the first step, a primary 14.1 MeV DT neutron knocks a triton or deuteron up to a spectrum of energies from 0 to more than 10 MeV. In the second step, the energetic knocked-on ion undergoes a DT reaction with a thermal ion, producing a neutron above the primary 14 MeV peak. Transport and energy loss of the knock-on ions inducing the RIF reactions directly affect the RIF production rate, and RIF measurements can be used to determine the stopping power for charged particles in the plasma. Here we present the formalism for extracting the stopping power from the measured RIF signals. We find that the stopping power extracted from these measurements is consistent with a strongly coupled quantum degenerate plasma for the high-density cold fuel surrounding the hotspot of the compressed capsule. These RIF measurements represent the first determination of stopping powers in strongly coupled plasmas. [Preview Abstract] |
Thursday, October 30, 2014 11:30AM - 12:00PM |
TI2.00005: Highly nonlinear ablative Rayleigh-Taylor experiments on the National Ignition Facility Invited Speaker: Alexis Casner Cryogenic indirect-drive implosions on NIF have evidenced that the ablative Rayleigh-Taylor Instability (RTI) is an important driver of hot spot mix [1]. This motivates the switch to a higher adiabat implosion design [2]. Academic tests in physical regimes not encountered in ICF will help to build a better understanding of hydrodynamic instabilities. Under the NIF Discovery Science program, indirect drive experiments were performed to study the ablative RTI in transition from weakly nonlinear to highly nonlinear regime [3]. The unique capabilities of the NIF are harnessed to accelerate planar samples over much larger distances ($\sim$ 2 mm) and longer time periods ($\sim$ 10 ns) than previously achieved. The existence of a turbulent-like regime at ablation front is in fact not precluded by theory. This question is crucial for laboratory astrophysics and supernova of type Ia explosions based on the analogy between the flame front and the ablation front. A modulated package is accelerated by a 180 eV radiative temperature plateau created by a room temperature gas-filled platform irradiated by 64 NIF beams. Simultaneous trajectory and RTI growth measurements are performed. We present measurements made for various two-dimensional patterns (single-mode and broadband multimode modulations) and compare our results with weakly nonlinear analytical theory and FCI2 hydrocode simulations. The dependence of RTI growth on initial conditions and ablative stabilization is emphasized, as well as the possibility of measuring RTI bubble-merger for the first time in indirect-drive. In collaboration with D. Martinez, V.A. Smalyuk, B. Remington (LLNL), L. Masse, S. Liberatore, P. Loiseau (CEA). \\[4pt] [1] S.P. Regan et al., Phys. Rev. Lett. 111, 045001 (2013).\\[0pt] [2] O.A. Hurricane et al., Nature 506, 343 (2014).\\[0pt] [3] A. Casner et al., Phys. Plasmas 19, 082708 (2012). [Preview Abstract] |
Thursday, October 30, 2014 12:00PM - 12:30PM |
TI2.00006: Computational modeling of Krypton Gas Puffs on Z Invited Speaker: Christopher Jennings Large diameter multi-shell gas puffs rapidly imploded by high current ($\sim$ 20MA, $\sim$ 100ns) on the Z generator of Sandia National Laboratories are able to produce high-intensity K-shell radiation. Experiments are currently underway to produce Krypton K-shell emission at $\sim$ 13keV, from double annular shell gas puffs imploded from a 12cm diameter onto a central gas jet. Efficiently radiating at these high photon energies represents a significant challenge which necessitates the careful design and optimization of the gas distribution. To facilitate this we hydro-dynamically model the gas flow out of the nozzle, before imploding that mass distribution using a 3-dimensional resistive, radiative MHD code (GORGON). We present details of how modeled gas profiles are validated against 2-dimensional interferometric measurements of the initial gas distribution, and MHD calculations are validated against power, yield, spectral and imaging diagnostics of the experiments. This approach has enabled us to iterate between modeling the implosion and gas flow from the nozzle to optimize radiative output from this combined system. Guided by our implosion calculations we have designed and implemented gas profiles that help mitigate disruption from Magneto-Rayleigh--Taylor implosion instabilities, while preserving sufficient kinetic energy to thermalize to the high temperatures required for K-shell emission. Predicted increases in yield from introducing a relief feature into the inner gas nozzle to create a radially increasing density distribution were recovered in experiment. K-shell yield is predicted to further increase by the introduction of an on-axis gas jet, although the mass of this jet must be carefully selected with respect to the delivered current to avoid reducing the yield. For Kr gas puffs the predicted K-shell yield increase from addition of a light central jet was realized in the experiments, considerably increasing the yield over previous results. Further confidence in our ability to model different gas profiles was added by comparisons with smaller diameter Ar gas puffs, where simulations reproduce the effect of a central jet for different gas profiles. [Preview Abstract] |
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