Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session BI3: ICF Implosions, Diagnostics, Laboratory Shocks |
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Chair: Charles Nakhleh, Sandia National Laboratories Room: Ballroom BC |
Monday, October 29, 2012 9:30AM - 10:00AM |
BI3.00001: Detailed Implosion Modeling of DT-Layered Experiments on the National Ignition Facility Invited Speaker: Daniel Clark Several dozen Inertial Confinement Fusion (ICF) implosion experiments with cryogenic DT layers have now been performed on the National Ignition Facility (NIF). Each of these yields a wealth of data: x-ray image shape and size, primary and down-scattered neutron image shape and size, neutron down-scatter fraction, burn-averaged ion temperature, neutron yield, etc. Compared to radiation-hydrodynamics simulations, however, the measured capsule yield is usually lower by a factor of five to ten, and the ion temperature varies from simulations, while most other observables are well matched between experiment and simulation. In an effort to understand this discrepancy, we perform detailed post-shot simulations of a subset of NIF implosion experiments. Using two-dimensional HYDRA simulations of the capsule only, these simulations represent as accurately as possible the conditions of a given experiment, including the as-shot capsule metrology, capsule surface roughness, and ice layer defects as seeds for the growth of hydrodynamic instabilities. The radiation drive used in these capsule-only simulations can be tuned to reproduce quite well the measured implosion timing, kinematics and low-mode asymmetry. In order to simulate the experiments as accurately as possible, a limited number of fully three-dimensional implosion simulations are also being performed. The post-shot simulation procedure and the ensemble of post-shot implosion simulations will be described, and the remaining discrepancies with the data discussed as they suggest the need for possible modifications to the physics models included in simulations or alternate directions for the experimental campaign. [Preview Abstract] |
Monday, October 29, 2012 10:00AM - 10:30AM |
BI3.00002: Integrated Diagnostic Analysis of ICF Capsule Performance Invited Speaker: Charles Cerjan An understanding of the dynamics of imploding Inertial Confinement Fusion (ICF) capsules is crucial to achieve high convergence and gain. The relative roles of laser irradiation, hohlraum drive, and capsule response are intertwined and will be difficult to disentangle unless appropriate diagnostic probes are fielded and their results correlated. In the case of capsule implosions, several currently deployed diagnostics provide important information about the size and shape of the developing hot spot through x-ray self-emission, neutron production and average ion temperature by neutron time-of-flight signals, shell material mix into the hot spot by high-resolution x-ray spectra, and remaining mass during convergent ablation by x-ray backlighting. Obtaining a physically consistent picture of the implosion dynamics requires an integration of these disparate experimental data. This talk describes a three-dimensional model that attempts this integration. Assuming pressure equilibrium at peak compression and invoking simple radiative and equation-of-state relations, the pressure, density and electron temperature are obtained by optimized fitting of the experimental output to simple, global functional forms. The fitting procedure is sufficiently flexible to incorporate typical observational data such as x-ray self-emission, neutron time-of-flight signals, neutron yield, high-resolution x-ray spectra and radiographic images. Once consistency is obtained, many important secondary quantities can be derived such as the fuel areal density, high energy x-ray emission, neutron images, and nuclear activation. This approach has been validated by comparison with radiation-hydrodynamic simulations, producing semi-quantitative agreement and is now routinely used to characterize cryogenic implosion experiments. This talk will provide an overview of the implementation of the model and describe its application to recent experimental data.\\[4pt] In addition to my collaborators Paul Springer and Scott Sepke, the author would like to thank many scientists for their assistance: J. Knauer, J. McNaney, M. Moran, D. Munro, G. Kyrala, D. Bradley, N. Izumi, T. Ma, S. Glenn, D. Clark, O. Jones, R. Town and S. Weber. This work was performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, October 29, 2012 10:30AM - 11:00AM |
BI3.00003: Nuclear imaging of implosions at the National Ignition Facility Invited Speaker: Gary Grim The nuclear diagnostic capability at the National Ignition Facility (NIF), includes: neutron imaging, providing images of where neutrons are produced and scattered; gamma reaction history, providing bang time and burn width; neutron time-of-flight and spectrometry, providing directional information on yield, ion temperature, and scattering; and nuclear activation, providing directional yield information. The set provides a self-consistent, nuclear picture of the fuel assembly during burn. Recent experiments indicate in a typical implosion the hot core is approximately 50 $\mu$m in diameter and enveloped by a dense shell $\sim$15 $\mu$m thick. The burn width, yield, and volume of the core indicate pressures of 75 to 100 Gbar are being achieved. Further, image, time-of-flight, and activation data indicate the shell is thicker on the poles than the equator by approximately 40\%. Comparison of the shell geometry data with time-of-flight scattering ratios indicate the density of the shell may be at least 40\% lower than would be obtained using the initial fuel payload, indicative of instability growth at the fuel-ablator interface, or possibly density gradients within the shell. We present a review of the current data and the status of fuel-assembly analyses based on these data. [Preview Abstract] |
Monday, October 29, 2012 11:00AM - 11:30AM |
BI3.00004: Neutron Spectroscopy on the National Ignition Facility Invited Speaker: J.P. Knauer The performance of cryogenic fuel implosion experiments in progress at the National Ignition Facility (NIF) is measured by an experimental threshold factor\footnote{M. J. Edwards \textit{et al}., Phys. Plasmas \textbf{18}, 051003 (2011).} (ITFX) and a generalized Lawson Criterion.\footnote{C. D. Zhou and R. Betti, Phys. Plasmas \textbf{15}, 102707 (2008); P. Y. Chang\textit{ et al.}, Phys. Rev. Lett. \textbf{104}, 135002 (2010); and R. Betti \textit{et al.}, Phys. Plasmas \textbf{17}, 058102 (2010).\par } The ITFX metric is determined by the fusion yield and the areal density of an assembled deuterium-tritium (DT) fuel mass. Typical neutron yields from NIF implosions are greater than 10$^{14}$ allowing the neutron energy spectrum to be measured with unprecedented precision. A NIF spectrum is composed of neutrons created by fusion (DT, DD, and TT reactions) and neutrons scattered by the dense, cold fuel layer. Neutron scattering is used to determine the areal density of a NIF implosion and is measured along four lines of sight by two neutron time-of-flight detectors, a neutron imaging system, and the magnetic recoil spectrometer. An accurate measurement of the instrument response function for these detectors allows for the routine production of neutron spectra showing DT fuel areal densities up to 1.3 g/cm$^{2}$. Spectra over neutron energies of 10 to 17 MeV show areal-density asymmetries of 20{\%} that are inconsistent with simulations. New calibrations and analyses have expended the spectral coverage down to energies less than the deuterium backscatter edge (1.5 MeV for 14 MeV neutrons). These data and analyses are presented along with a compilation of other nuclear diagnostic data that show a larger-than-expected variation in the areal density over the cold fuel mass. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No DE-FC52-08NA28302. In collaboration with NIC. [Preview Abstract] |
Monday, October 29, 2012 11:30AM - 12:00PM |
BI3.00005: The evolution of a radiative shock system in a high-energy-density regime Invited Speaker: Carolyn Kuranz Radiative shocks, which are in a regime where most of the incoming energy flux is converted into radiation, occur in astrophysical systems as well as inertial confinement fusion experiments. This type of shock can be created in a laboratory using a high-powered laser. We have performed experiments on the Omega laser facility that irradiate a 20 $\mu $m thick Be disk with about 4 kJ of laser energy in a 1 ns pulse for an overall laser irradiance of $\sim $10$^{15}$ W/cm$^{2}$. The ablation pressure creates a 40 Mbar shock in the Be, which breaks out into Xe or Ar gas at 1.1 atm. The gas is shocked and accelerated and can reach velocities of over 130 km/s. At such high velocities the radiative fluxes become significant, which leads to extensive radiative cooling. The cooling of the shocked material causes compressions of about 20, which are higher than the typical hydrodynamic shock. Diagnostics for this experiment have included streaked and imaging x-ray radiography, optical pyrometry, VISAR and x-ray Thomson scattering. Experimental results to be presented include observations ranging from shock breakout of the Be disk at about 0.5 ns until 26 ns after the laser pulse is initiated. The data will be compared to results from the 3D radiation-hydrodynamic code developed at our Center for Radiative Shock Hydrodynamics. A thorough understanding of the uncertainties in these data is important to our project; these will be discussed. This work is funded by the Predictive Sciences Academic Alliances Program in NNSA-ASC via grant DEFC52- 08NA28616, by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-FG52-09NA29548, and by the National Laser User Facility Program, grant number~DE-NA0000850. [Preview Abstract] |
Monday, October 29, 2012 12:00PM - 12:30PM |
BI3.00006: Collisionless shocks and particle acceleration in laser-driven laboratory plasmas Invited Speaker: Frederico Fiuza Collisionless shocks are pervasive in space and astrophysical plasmas, from the Earth's bow shock to Gamma Ray Bursters; however, the microphysics underlying shock formation and particle acceleration in these distant sites is not yet fully understood. Mimicking these extreme conditions in laboratory is a grand challenge that would allow for a better understanding of the physical processes involved. Using \textit{ab initio} multi-dimensional particle-in-cell simulations, shock formation and particle acceleration are investigated for realistic laboratory conditions associated with the interaction of intense lasers with high-energy-density plasmas. Weibel-instability-mediated shocks are shown to be driven by the interaction of an ultraintense laser with overcritical plasmas. In this piston regime, the laser generates a relativistic flow that is Weibel unstable. The strong Weibel magnetic fields deflect the incoming flow, compressing it, and forming a shock. The resulting shock structure is consistent with previous simulations of relativistic astrophysical shocks, demonstrating for the first time the possibility of recreating these structures in laboratory. As the laser intensity is decreased and near-critical density plasmas are used, electron heating dominates over radiation pressure and electrostatic shocks can be formed. The electric field associated with the shock front can reflect ions from the background accelerating them to high energies. It is shown that high quality 200 MeV proton beams, required for tumor therapy, can be generated by using an exponentially decaying plasma profile to control competing accelerating fields. These results pave the way for the experimental exploration of space and astrophysical relevant shocks and particle acceleration with current laser systems. [Preview Abstract] |
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