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 KI2: Warm/Dense |
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Chair: Seigfried Glenzer, SLAC Room: 102ABC |
Tuesday, October 24, 2017 3:00PM - 3:30PM |
KI2.00001: Fluorescence and absorption spectroscopy for warm dense matter studies and ICF plasma diagnostics Invited Speaker: Stephanie Hansen The burning core of an inertial confinement fusion (ICF) plasma at stagnation is surrounded by a shell of warm, dense matter whose properties are difficult both to model (due to a complex interplay of thermal, degeneracy, and strong coupling effects) and to diagnose (due to low emissivity and high opacity). We demonstrate a promising technique to study the warm dense shells of ICF plasmas based on the fluorescence emission of dopants or impurities in the shell material. This emission, which is driven by x-rays produced in the hot core, exhibits signature changes in response to compression and heating. High-resolution measurements of absorption and fluorescence features can refine our understanding of the electronic structure of material under high compression, improve our models of density-driven phenomena such as ionization potential depression and plasma polarization shifts, and help diagnose shell density, temperature, mass distribution, and residual motion in ICF plasmas at stagnation. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. This work was supported by the U.S. Department of Energy, Office of Science Early Career Research Program, Office of Fusion Energy Sciences under FWP-14-017426. [Preview Abstract] |
Tuesday, October 24, 2017 3:30PM - 4:00PM |
KI2.00002: Warm Dense Matter Demonstrating Non-Drude Conductivity from Observations of Nonlinear Plasmon Damping Invited Speaker: Bastian B. L. Witte The thermal and electrical conductivity, equation of state and the spectral opacity in warm dense matter (WDM) are essential properties for modeling, e.g., fusion experiments or the magnetic field generation in planets. In the last decade it has been shown that x-ray Thomson scattering (XRTS) is an effective tool to determine plasma parameters like temperature and density in the WDM regime\footnote{Redmer and Glenzer, Rev. Mod. Phys. 81, 1625 (2009)}. Recently, the electrical conductivity was extracted from XRTS experiments for the first time\footnote{Sperling et al., Phys. Rev. Lett. 115, 115001 (2015)}. The spectrally resolved scattering data of aluminum, isochorically heated by the Linac Coherent Light Source (LCLS), show strong dependence on electron correlations. Therefore, the damping of plasmons, the collective electron oscillations, has to be treated beyond perturbation theory. \newline We present results for the dynamic transport properties in warm dense aluminum using density-functional-theory molecular dynamics (DFT-MD) simulations. The choice of the exchange-correlation (XC) functional, describing the interactions in the electronic subsystem, has significant impact on the ionization energy of bound electrons and the dynamic dielectric function. Our newly developed method for the calculation of XRTS signals including plasmon and bound-free transitions is based on transition matrix elements together with ionic contributions using uniquely DFT-MD simulations. The results show excellent agreement with the LCLS data if hybrid functionals are applied\footnote{Witte et al., Phys. Rev. Lett. 118, 225001 (2017)}. The experimental finding of nonlinear plasmon damping is caused by the non-Drude conductivity in warm dense aluminum. Here, we show further validation by comparing with x-ray absorption data. These findings enable new insights into the impact of XC functionals on calculated properties of WDM and allow detailed predictions for future experiments at the unprecedented densities on the NIF. [Preview Abstract] |
Tuesday, October 24, 2017 4:00PM - 4:30PM |
KI2.00003: Observation of a New High-Pressure Solid Phase in Dynamically Compressed Aluminum Invited Speaker: D.N. Polsin Aluminum is ideal for testing theoretical first-principles calculations because of the relative simplicity of its atomic structure. Density functional theory (DFT) calculations predict that Al transforms from an ambient-pressure, face-centered-cubic (fcc) crystal to the hexagonal close-packed (hcp) and body-centered-cubic (bcc) structures as it is compressed. Laser-driven experiments performed at the University of Rochester's Laboratory for Laser Energetics and the National Ignition Facility (NIF) ramp compressed Al samples to pressures up to 540 GPa without melting. Nanosecond \textit{in-situ} x-ray diffraction was used to directly measure the crystal structure at pressures where the solid--solid phase transformations of Al are predicted to occur. Laser velocimetry provided the pressure in the Al. Our results show clear evidence of the fcc--hcp and hpc--bcc transformations at $216\pm 9$ GPa and $321\pm 12$ GPa, respectively. This is the first experimental \textit{in-situ} observation of the bcc phase in compressed Al and a confirmation of the fcc--hcp transition previously observed under static compression at 217 GPa. The observations indicate these solid--solid phase transitions occur on the order of tens of nanoseconds time scales. In the fcc--hcp transition we find the original texture of the sample is preserved; however, the hcp--bcc transition diminishes that texture producing a structure that is more polycrystalline. The importance of this dynamic is discussed. The NIF results are the first demonstration of x-ray diffraction measurements at two different pressures in a single laser shot. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 24, 2017 4:30PM - 5:00PM |
KI2.00004: Visualization of the ultrafast structural phase transitions in warm dense matter. Invited Speaker: Mianzhen Mo It is still a great challenge to obtain real-time atomistic-scale information on the structural phase transitions that lead to warm dense matter state. Recent advances in ultrafast electron diffraction (UED) techniques [1] have opened up exciting prospects to unravel the mechanisms of solid-liquid phase transitions under these extreme non-equilibrium conditions. Here we report on precise measurements of melt time dependency on laser excitation energy density that resolve for the first time the transition from heterogeneous to homogeneous melting. This transition appears in both polycrystalline and single-crystal gold nanofilms with distinct measurable differences. These results test predictions from molecular-dynamics simulations with different interatomic potential models [2-3]. These data further deliver accurate structure factor data to large wavenumbers that allow us to constrain electron-ion equilibration constants. Our results demonstrate electron-phonon coupling strength much weaker than DFT calculations [4], and contrary to previous results [5], provide evidence for bond softening. [1] M. Mo, et al. RSI 87,11D810 (2016). [2] S. Mazevet, et al. PRL 95, 085002 (2005). [3] Z. Lin, et al. PRB 73, 184113 (2006). [4] Z. Lin, et al. PRB 77, 075133 (2008). [5] R. Ernstofer, et al. Science 323, 1033 (2009). [Preview Abstract] |
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