72 research outputs found
Comparison of Branching Ratio and Sum-Rule Analyses of Magnetic Circular Dichroism in X-Ray-Absorption Spectroscopy
Two localized picture methods of analyzing the magnetic circular dichroism in x-ray absorption will be applied to experimental results: the branching ratio (BR) and sum rule (SR) approaches. A derivation of the BR formulas and detailed comparison to the SR expressions will be made, including error estimations. The BR approach will be seen to be a limiting case form of the SR spin-moment expression and provide a simple picture of the underlying physics in magnetic x-ray circular dichroism absorption in 3d magnetic materials
Carbon ionization at Gbar pressures: an ab initio perspective on astrophysical high-density plasmas
A realistic description of partially-ionized matter in extreme thermodynamic
states is critical to model the interior and evolution of the multiplicity of
high-density astrophysical objects. Current predictions of its essential
property, the ionization degree, rely widely on analytical approximations that
have been challenged recently by a series of experiments. Here, we propose a
novel ab initio approach to calculate the ionization degree directly from the
dynamic electrical conductivity using the Thomas-Reiche-Kuhn sum rule. This
Density Functional Theory framework captures genuinely the condensed matter
nature and quantum effects typical for strongly-correlated plasmas. We
demonstrate this new capability for carbon and hydrocarbon, which most notably
serve as ablator materials in inertial confinement fusion experiments aiming at
recreating stellar conditions. We find a significantly higher carbon ionization
degree than predicted by commonly used models, yet validating the qualitative
behavior of the average atom model Purgatorio. Additionally, we find the carbon
ionization state to remain unchanged in the environment of fully-ionized
hydrogen. Our results will not only serve as benchmark for traditional models,
but more importantly provide an experimentally accessible quantity in the form
of the electrical conductivity.Comment: accepted for publication in Physical Review Researc
Theoretical and experimental investigation of the equation of state of boron plasmas
We report a theoretical equation of state (EOS) table for boron across a wide
range of temperatures (5.110-5.210 K) and densities
(0.25-49 g/cm), and experimental shock Hugoniot data at unprecedented high
pressures (5608118 GPa). The calculations are performed with full,
first-principles methods combining path integral Monte Carlo (PIMC) at high
temperatures and density functional theory molecular dynamics (DFT-MD) methods
at lower temperatures. PIMC and DFT-MD cross-validate each other by providing
coherent EOS (difference 1.5 Hartree/boron in energy and 5% in pressure)
at 5.110 K. The Hugoniot measurement is conducted at the National
Ignition Facility using a planar shock platform. The pressure-density relation
found in our shock experiment is on top of the shock Hugoniot profile predicted
with our first-principles EOS and a semi-empirical EOS table (LEOS 50). We
investigate the self diffusivity and the effect of thermal and pressure-driven
ionization on the EOS and shock compression behavior in high pressure and
temperature conditions We study the performance sensitivity of a polar
direct-drive exploding pusher platform to pressure variations based on
comparison of the first-principles calculations with LEOS 50 via 1D
hydrodynamic simulations. The results are valuable for future theoretical and
experimental studies and engineering design in high energy density research.
(LLNL-JRNL-748227)Comment: 12 pages, 9 figures, 2 table
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