169 research outputs found
Evolution of electronic and ionic structure of Mg-clusters with the growth cluster size
The optimized structure and electronic properties of neutral and singly
charged magnesium clusters have been investigated using ab initio theoretical
methods based on density-functional theory and systematic post-Hartree-Fock
many-body perturbation theory accounting for all electrons in the system. We
have systematically calculated the optimized geometries of neutral and singly
charged magnesium clusters consisting of up to 21 atoms, electronic shell
closures, binding energies per atom, ionization potentials and the gap between
the highest occupied and the lowest unoccupied molecular orbitals. We have
investigated the transition to the hcp structure and metallic evolution of the
magnesium clusters, as well as the stability of linear chains and rings of
magnesium atoms. The results obtained are compared with the available
experimental data and the results of other theoretical works.Comment: 30 pages, 10 figures, 3 table
Development of an x-ray radiography platform to study laser-direct-drive energy coupling at the National Ignition Facility
Developing a platform for Fresnel diffractive radiography with 1 ÎŒm spatial resolution at the National Ignition Facility
Indications of flow near maximum compression in layered deuterium-tritium implosions at the National Ignition Facility
An accurate understanding of burn dynamics in implosions of cryogenically layered deuterium (D) and tritium (T) filled capsules, obtained partly through precision diagnosis of these experiments, is essential for assessing the impediments to achieving ignition at the National Ignition Facility. We present measurements of neutrons from such implosions. The apparent ion temperatures T[subscript ion] are inferred from the variance of the primary neutron spectrum. Consistently higher DT than DD T[subscript ion] are observed and the difference is seen to increase with increasing apparent DT T[subscript ion]. The line-of-sight rms variations of both DD and DT T[subscript ion] are small, ⌠150 eV, indicating an isotropic source. The DD neutron yields are consistently high relative to the DT neutron yields given the observed T[subscript ion]. Spatial and temporal variations of the DT temperature and density, DD-DT differential attenuation in the surrounding DT fuel, and fluid motion variations contribute to a DT T[subscript ion] greater than the DD T[subscript ion], but are in a one-dimensional model insufficient to explain the data. We hypothesize that in a three-dimensional interpretation, these effects combined could explain the results.Lawrence Livermore National Laboratory (Contract No. DE-AC52- 07NA27344
High-pressure chemistry of hydrocarbons relevant to planetary interiors and inertial confinement fusion
Diamond formation in polystyrene (C8H8)n, which is laser-compressed and heated to conditions around 150âGPa and 5000âK, has recently been demonstrated in the laboratory [Kraus et al., Nat. Astron. 1, 606â611 (2017)]. Here, we show an extended analysis and comparison to first-principles simulations of the acquired data and their implications for planetary physics and inertial confinement fusion. Moreover, we discuss the advanced diagnostic capabilities of adding high-quality small angle X-ray scattering and spectrally resolved X-ray scattering to the platform, which shows great prospects of precisely studying the kinetics of chemical reactions in dense plasma environments at pressures exceeding 100âGPa
Improving a high-efficiency, gated spectrometer for x-ray Thomson scattering experiments at the National Ignition Facility
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Recovery of release cloud from laser shock-loaded graphite and hydrocarbon targets: in search of diamonds
This work presents first insights into the dynamics of free-surface release clouds from dynamically compressed polystyrene and pyrolytic graphite at pressures up to 200 GPa, where they transform into diamond or lonsdaleite, respectively. These ejecta clouds are released into either vacuum or various types of catcher systems, and are monitored with high-speed recordings (frame rates up to 10 MHz). Molecular dynamics simulations are used to give insights to the rate of diamond preservation throughout the free expansion and the catcher impact process, highlighting the challenges of diamond retrieval. Raman spectroscopy data show graphitic signatures on a catcher plate confirming that the shock-compressed PS is transformed. First electron microscopy analyses of solid catcher plates yield an outstanding number of different spherical-like objects in the size range between ten(s) up to hundreds of nanometres, which are one type of two potential diamond candidates identified. The origin of some objects can unambiguously be assigned, while the history of others remains speculative
Observations of Continuum Depression in Warm Dense Matter with X-Ray Thomson Scattering
Imaging of high-energy x-ray emission from cryogenic thermonuclear fuel implosions on the NIF
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