Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session TI3: HEDP and Laboratory Astrophysics |
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Chair: Carolyn Kuranz, University of Michigan Room: Ballroom AC |
Thursday, November 17, 2011 9:30AM - 10:00AM |
TI3.00001: Plasma Gradient Piston: a new approach to precision pulse shaping Invited Speaker: We have successfully developed a method to create shaped pressure drives from large shocks that can be applied to a wide variety of experimental platforms. The method consists of transforming a large shock or blast wave into a ramped pressured drive by utilizing a graded density reservoir that unloads across a gap and stagnates against the sample being studied. The utilization of a graded density reservoir, different materials, and a gap transforms the energy in the initial large shock into a quasi-isentropic ramped compression. Control of the ramp history is via the size of the initial shock, the chosen reservoir materials, their densities, the thickness of each density layer, and the gap size. There are two keys to utilizing this approach to create ramped drives: the ability to produce a large shock, and making the layered density reservoir. A number of facilities can produce the strong initial shock (Z, Omega, NIF, Phoenix, high explosives, NIKE, LMJ, pulsed power, {\ldots}). We have demonstrated ramped drives from 0.5 to 1.5 Mbar utilizing a large shock created at the Omega laser facility. We recently concluded a pair of NIF drive shots where we successfully converted a hohlraum-generated shock into a stepped, ramped pressure drive with a peak pressure of $\sim $4 - 5 Mbar in a Ta sample. We will explain the basic concepts needed for producing a ramped pressure drive, compare experimental data with simulations from Omega (P$_{max}\sim $ 1 Mbar) and NIF (P$_{max}\sim $ 5-10 Mbar), and present designs for ramped, staged-shock designs up to P$_{max} \quad \sim $ 30 Mbar. The approach that we have developed enables precision pulse shaping of the drive (applied pressure vs. time) via target characteristics, as opposed to tailoring laser power vs time or Z-pinch facility current vs time. This enables ramped, quasi-isentropic materials studies to be performed on a wide variety of HED facilities. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-490532. [Preview Abstract] |
Thursday, November 17, 2011 10:00AM - 10:30AM |
TI3.00002: Refractive Index of Lithium Fluoride Ramp Compressed to 800 GPa Invited Speaker: The compression of materials to high pressure can alter their optical properties in ways that provide insight into the resulting structural changes. Under strong shock compression, transparent insulators transform into conducting fluids as a result of pressure-induced reduction of the band gap and thermal promotion of electrons across that gap. LiF is ramp compressed to 800 GPa on the Omega Laser Facility without generating shocks, producing high pressures at significantly lower temperatures than would be created by shock waves. Ramp compressed lithium fluoride (LiF) is observed to remain transparent to 800 GPa, pressures seven times higher than previous shock compression experiments. The ramp-compressed refractive index of LiF is measured at pressures up to 800 GPa and depends linearly on density. This is the highest-pressure refractive index measurement made to date. The linear dependence of the refractive index and density is examined using a single oscillator model. This model indicates that the linear behavior is a result of monoatomic closure of the band gap. Extrapolation of these results indicates that the band gap closure (metallization) will be greater than 4,000 GPa. The high metallization pressure of LiF is attributed to its large band gap and isoelectronic counterparts that exhibit high metallization pressures. The observed high-pressure transparency and measurement of LiF refractive index enables advancement of in situ experiments to higher-pressure regimes. In collaboration with T.R. Boehly (LLE), M.A. Barrios (LLE -- now at LLNL), D.D. Meyerhofer (LLE), J.H. Eggert (LLNL), R.F. Smith (LLNL), D.G. Hicks (LLNL), P.M. Celliers (LLNL), and G.W. Collins (LLNL). [Preview Abstract] |
Thursday, November 17, 2011 10:30AM - 11:00AM |
TI3.00003: Shock Compression of Liquid Noble Gases to Multi-Mbar Pressures Invited Speaker: The high pressure -- high temperature behavior of noble gases is of considerable interest because of their use in z-pinch liners for fusion studies and for understanding astrophysical and planetary evolution. However, our understanding of the equation of state (EOS) of the noble gases at extreme conditions is limited. A prime example of this is the liquid xenon Hugoniot. Previous EOS models rapidly diverged on the Hugoniot above 1 Mbar because of differences in the treatment of the electronic contribution to the free energy. Similar divergences are observed for krypton EOS. Combining shock compression experiments and density functional theory (DFT) simulations, we can determine the thermo-physical behavior of matter under extreme conditions. The experimental and DFT results have been instrumental to recent developments in planetary astrophysics and inertial confinement fusion. Shock compression experiments are performed using Sandia's Z-Accelerator to determine the Hugoniot of liquid xenon and krypton in the Mbar regime. Under strong pressure, krypton and xenon undergo an insulator to metal transition. In the metallic state, the shock front becomes reflective allowing for a direct measurement of the sample's shock velocity using laser interferometry. The Hugoniot state is determined using a Monte Carlo analysis method that accounts for systematic error in the standards and for correlations. DFT simulations at these extreme conditions show good agreement with the experimental data -- demonstrating the attention to detail required for dealing with elements with relativistic core states and d-state electrons. The results from shock compression experiments and DFT simulations are presented for liquid xenon to 840 GPa and for liquid krypton to 800 GPa, decidedly increasing the range of known behavior of both gases. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U. S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, November 17, 2011 11:00AM - 11:30AM |
TI3.00004: Dynamic compression of water to 700 GPa: single- and double shock experiments on Sandia's Z machine, first principles simulations, and structure of water planets Invited Speaker: Significant progress has over the last few years been made in high energy density physics (HEDP) by executing high-precision multi-Mbar experiments and performing first-principles simulations for elements ranging from carbon [1] to xenon [2]. The properties of water under HEDP conditions are of particular importance in planetary science due to the existence of ice-giants like Neptune and Uranus. Modeling the two planets, as well as water-rich exoplanets, requires knowing the equation of state (EOS), the pressure as a function of density and temperature, of water with high accuracy. Although extensive density functional theory (DFT) simulations have been performed for water under planetary conditions [3] experimental validation has been lacking. Accessing thermodynamic states along planetary isentropes in dynamic compression experiments is challenging because the principal Hugoniot follows a significantly different path in the phase diagram. In this talk, we present experimental data for dynamic compression of water up to 700 GPa, including in a regime of the phase-diagram intersected by the Neptune isentrope and water-rich models for the exoplanet GJ436b. The data was obtained on the Z-accelerator at Sandia National Laboratories by performing magnetically accelerated flyer plate impact experiments measuring both the shock and re-shock in the sample. The high accuracy makes it possible for the data to be used for detailed model validation: the results validate first principles based thermodynamics as a reliable foundation for planetary modeling and confirm the fine effect of including nuclear quantum effects on the shock pressure. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. \\[4pt] [1] M.D. Knudson, D.H. Dolan, and M.P. Desjarlais, SCIENCE 322, 1822 (2008).\\[0pt] [2] S. Root, et al., Phys. Rev. Lett. 105, 085501 (2010).\\[0pt] [3] M. French, et al., Phys. Rev. B 79, 054107 (2009). [Preview Abstract] |
Thursday, November 17, 2011 11:30AM - 12:00PM |
TI3.00005: Ramp Compression of Carbon above 50 Mbar on NIF Invited Speaker: The National Ignition Facility (NIF) offers unprecedented opportunities to push the limits of condensed-matter and materials physics. By using ramp-compression techniques on NIF we have generated pressures into the multi-TPa regime on carbon and 1 TPa on iron. Our multiple step samples are driven from hohlraum x-radiation with a characteristic radiation temperature history determined by our laser drive: 176 beams with 0.4-1MJ of energy in a ramp laser profile up to 30ns in duration. We employ Lagrangian sound speed analysis to determine high-pressure compressibility and information on dynamic strength and the existence of high-pressure structural phase transformations. These experiments represent an order of magnitude increase in the maximum pressure achievable under ramp compression and generate conditions relevant to the core states of the giant planets. I will report on these experiments, and plans for upcoming x-ray diffraction experiments on NIF [Preview Abstract] |
Thursday, November 17, 2011 12:00PM - 12:30PM |
TI3.00006: Studying astrophysical collisionless shocks with high-power laser experiments Invited Speaker: Shocks in astrophysics are ubiquitous, occurring in supernovae, gamma ray bursts, and protostellar jets. In a broad range of low density astrophysical plasmas, the ion-ion collision mean free path is typically very large compared to the relevant spatial scales. Hence, when shocks form they are typically collisionless, resulting from plasma instabilities and self-generated magnetic fields. High power laser experiments can achieve the conditions necessary for the formation of collisionless shocks, allowing laboratory studies of this unique nonlinear physics. The experiments are aimed at probing the importance of the current filamentation (Weibel) instability and external magnetic field in collisionless shock formation. A series of Omega experiments have begun using high velocity interpenetrating plasma flows to create collisionless shocks. Single and double CH$_2$ foils have been irradiated with a laser intensity of $\sim$10$^{16}$ W/cm$^2$. The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2,000 km/s, an electron temperature of $\sim$110 eV, an ion temperature of $\sim$30 eV, and a density of $\sim$10$^{18}$ cm$^{-3}$ were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured double foil plasma conditions were used to calculate the Coulomb mean free path, $\lambda_{mfp}$ $\sim$30 mm, and the ion skin depth, c/$\omega_{pi}$ $\sim$0.12 mm, with an interaction length, L, of $\sim$8 mm. With c/$\omega_{pi}$ $<$ L $<$ $\lambda_{mfp}$ we are in a regime where collisionless shock formation is possible. Experimental results will be shown and compared with simulations and theory. Planned follow-on NIF experiments will be described. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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