335 research outputs found
XUV Opacity of Aluminum between the Cold-Solid to Warm-Plasma Transition
We present calculations of the free-free XUV opacity of warm, solid-density
aluminum at photon energies between the plasma frequency at 15 eV and the
L-edge at 73 eV, using both density functional theory combined with molecular
dynamics and a semi-analytical model in the RPA framework with the inclusion of
local field corrections. As the temperature is increased from room temperature
to 10 eV, with the ion and electron temperatures equal, we calculate an
increase in the opacity in the range over which the degree of ionization is
constant. The effect is less pronounced if only the electron temperature is
allowed to increase. The physical significance of these increases is discussed
in terms of intense XUV-laser matter interactions on both femtosecond and
picosecond time-scales.Comment: 4 pages, 3 figure
Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium
We have performed quantum molecular dynamics simulations for dense helium to
study the nonmetal-to-metal transition at high pressures. We present new
results for the equation of state and the Hugoniot curve in the warm dense
matter region. The optical conductivity is calculated via the Kubo-Greenwood
formula from which the dc conductivity is derived. The nonmetal-to-metal
transition is identified at about 1 g/ccm. We compare with experimental results
as well as with other theoretical approaches, especially with predictions of
chemical models.Comment: 4 pages, 5 figure
Probing the interiors of the ice giants: Shock compression of water to 700 GPa and 3.8 g/ccm
Recently there has been tremendous increase in the number of identified
extra-solar planetary systems. Our understanding of their formation is tied to
exoplanet internal structure models, which rely upon equations of state of
light elements and compounds like water. Here we present shock compression data
for water with unprecedented accuracy that shows water equations of state
commonly used in planetary modeling significantly overestimate the
compressibility at conditions relevant to planetary interiors. Furthermore, we
show its behavior at these conditions, including reflectivity and isentropic
response, is well described by a recent first-principles based equation of
state. These findings advocate this water model be used as the standard for
modeling Neptune, Uranus, and "hot Neptune" exoplanets, and should improve our
understanding of these types of planets.Comment: Accepted to Phys. Rev. Lett.; supplementary material attached
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Wire array z-pinch insights for high X-ray power generation
The discovery that the use of very large numbers of wires enables high x-ray power to be generated from wire-array z-pinches represents a breakthrough in load design for large pulsed power generators, and has permitted high temperatures to be generated in radiation cavities on Saturn and Z. In this paper, changes in x-ray emission characteristics as a function of wire number, array mass, and load radius, for 20-mm-long aluminum arrays on Saturn that led to these breakthrough hohlraum results, are discussed and compared with a few related emission characteristics of high-wire-number aluminum and tungsten arrays on Z. X-ray measurement comparisons with analytic models and 2-D radiation-magnetohydrodynamic (RMHC) code simulations in the x-y and r-z planes provide confidence in the ability of the models and codes to predict future x-ray performance with very-large-number wire arrays
Anthropology, Brokerage and Collaboration in the development of a Tongan Public Psychiatry: Local Lessons for Global Mental Health
The Global Mental Health (GMH) movement has revitalised questions of the translatability of psychiatric concepts and the challenges of community engagement in countries where knowledge of the biomedical basis for psychiatric diagnosis is limited or challenged by local cultural codes. In Tonga, the local psychiatrist Dr Puloka has successfully established a publicly accessible psychiatry that has raised admission rates for serious mental illness and addressed some of the stigma attached to diagnosis. On the basis of historical analysis and ethnographic fieldwork with healers, doctors and patients since 1998, this article offers an ethnographic contextualization of the development and reception of three key interventions during the 1990s inspired by traditional healing and reliant on the translation of psychiatric terms and diagnosis. Dr Puloka’s use of medical anthropological and transcultural psychiatry research informed a community engaged brokerage between the implications of psychiatric nosologies and local needs. As such it reveals deficiencies in current polarised positions on the GMH project and offers suggestions to address current challenges of the Global Mental Health movement
Role of lattice structure and low temperature resistivity on fast electron beam filamentation in carbon
The influence of low temperature (eV to tens-of-eV) electrical resistivity on the onset of the filamentation instability in fast-electron transport is investigated in targets comprising of layers of ordered (diamond) and disordered (vitreous) carbon. It is shown experimentally and numerically that the thickness of the disordered carbon layer influences the degree of filamentation of the fast-electron beam. Strong filamentation is produced if the thickness is of the order of 60 μm or greater, for an electron distribution driven by a sub-picosecond, mid-1020 Wcm-2 laser pulse. It is shown that the position of the vitreous carbon layer relative to the fast-electron source (where the beam current density and background temperature are highest) does not have a strong effect because the resistive filamentation growth rate is high in disordered carbon over a wide range of temperatures up to the Spitzer regime
Annular fast electron transport in silicon arising from low-temperature resistivity
Fast electron transport in Si, driven by ultra-intense laser pulses, is investigated experimentally and via 3D hybrid-PIC simulations. A transition from a Gaussian-like to an annular fast electron beam profile is demonstrated and explained by resistively generated magnetic fields. The results highlight the potential to completely transform the beam transport pattern by tailoring the resistivity-temperature profile at temperatures as low as a few eV
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