116 research outputs found
A Fluid Dynamics Calculation of Sputtering from a Cylindrical Thermal Spike
The sputtering yield, Y, from a cylindrical thermal spike is calculated using
a two dimensional fluid dynamics model which includes the transport of energy,
momentum and mass. The results show that the high pressure built-up within the
spike causes the hot core to perform a rapid expansion both laterally and
upwards. This expansion appears to play a significant role in the sputtering
process. It is responsible for the ejection of mass from the surface and causes
fast cooling of the cascade. The competition between these effects accounts for
the nearly linear dependence of with the deposited energy per unit depth
that was observed in recent Molecular Dynamics simulations. Based on this we
describe the conditions for attaining a linear yield at high excitation
densities and give a simple model for this yield.Comment: 10 pages, 9 pages (including 9 figures), submitted to PR
Crater formation by fast ions: comparison of experiment with Molecular Dynamics simulations
An incident fast ion in the electronic stopping regime produces a track of
excitations which can lead to particle ejection and cratering. Molecular
Dynamics simulations of the evolution of the deposited energy were used to
study the resulting crater morphology as a function of the excitation density
in a cylindrical track for large angle of incidence with respect to the surface
normal. Surprisingly, the overall behavior is shown to be similar to that seen
in the experimental data for crater formation in polymers. However, the
simulations give greater insight into the cratering process. The threshold for
crater formation occurs when the excitation density approaches the cohesive
energy density, and a crater rim is formed at about six times that energy
density. The crater length scales roughly as the square root of the electronic
stopping power, and the crater width and depth seem to saturate for the largest
energy densities considered here. The number of ejected particles, the
sputtering yield, is shown to be much smaller than simple estimates based on
crater size unless the full crater morphology is considered. Therefore, crater
size can not easily be used to estimate the sputtering yield.Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see:
http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded
5/16/01, with minor text changes + new figure
Coulomb Explosion and Thermal Spikes
A fast ion penetrating a solid creates a track of excitations. This can
produce displacements seen as an etched track, a process initially used to
detect energetic particles but now used to alter materials. From the seminal
papers by Fleischer et al. [Phys. Rev. 156, 353 (1967)] to the present [C.
Trautmann, S. Klaumunzer and H. Trinkaus, Phys. Rev. Lett. 85, 3648 (2000)],
`Coulomb explosion' and thermal spike models are treated as conflicting models
for describing ion track effects. Here molecular dynamics simulations of
electronic-sputtering, a surface manifestation of ion track formation, show
that `Coulomb explosion' produces a `heat' spike so that these are early and
late aspects of the same process. Therefore, differences in scaling are due to
the use of incomplete spike models.Comment: Submitted to PRL. 4 pages, 3 figures. For related movies see:
http://dirac.ms.virginia.edu/~emb3t/coulomb/coulomb.html PACS added in new
versio
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Simulations of carbon sputtering in fusion reactor divertor plates
The interaction of edge plasma with material surfaces raises key issues for the viability of the International Thermonuclear Reactor (ITER) and future fusion reactors, including heat-flux limits, net material erosion, and impurity production. After exposure of the graphite divertor plate to the plasma in a fusion device, an amorphous C/H layer forms. This layer contains 20-30 atomic percent D/T bonded to C. Subsequent D/T impingement on this layer produces a variety of hydrocarbons that are sputtered back into the sheath region. We present molecular dynamics (MD) simulations of D/T impacts on amorphous carbon layer as a function of ion energy and orientation, using the AIREBO potential. In particular, energies are varied between 10 and 150 eV to transition from chemical to physical sputtering. These results are used to quantify yield, hydrocarbon composition and eventual plasma contamination
Cultural Expertise and Socio-legal Studies: Introduction
International audienceEmerald is a global publisher linking research and practice to the benefit of society. The company manages a portfolio of more than 290 journals and over 2,350 books and book series volumes, as well as providing an extensive range of online products and additional customer resources and services. Emerald is both COUNTER 4 and TRANSFER compliant. The organization is a partner of the Committee on Publication Ethics (COPE) and also works with Portico and the LOCKSS initiative for digital archive preservation
Dust amorphization in protoplanetary disks
High-energy irradiation of the circumstellar material might impact the
structure and the composition of a protoplanetary disk and hence the process of
planet formation. In this paper, we present a study on the possible influence
of the stellar irradiation, indicated by X-ray emission, on the crystalline
structure of the circumstellar dust. The dust crystallinity is measured for 42
class II T Tauri stars in the Taurus star-forming region using a decomposition
fit of the 10 micron silicate feature, measured with the Spitzer IRS
instrument. Since the sample includes objects with disks of various
evolutionary stages, we further confine the target selection, using the age of
the objects as a selection parameter. We correlate the X-ray luminosity and the
X-ray hardness of the central object with the crystalline mass fraction of the
circumstellar dust and find a significant anti-correlation for 20 objects
within an age range of approx. 1 to 4.5 Myr. We postulate that X-rays represent
the stellar activity and consequently the energetic ions of the stellar winds
which interact with the circumstellar disk. We show that the fluxes around 1 AU
and ion energies of the present solar wind are sufficient to amorphize the
upper layer of dust grains very efficiently, leading to an observable reduction
of the crystalline mass fraction of the circumstellar, sub-micron sized dust.
This effect could also erase other relations between crystallinity and
disk/star parameters such as age or spectral type.Comment: accepted for publication by A&
The radial distribution of dust species in young brown dwarf disks
We present a study of the radial distribution of dust species in young brown
dwarf disks. Our work is based on a compositional analysis of the 10 and 20
micron silicate emission features for brown dwarfs in the Taurus-Auriga
star-forming region. A fundamental finding of our work is that brown dwarfs
exhibit stronger signs of dust processing in the cold component of the disk,
compared to the higher mass T Tauri stars in Taurus. For nearly all of our
targets, we find a flat disk structure, which is consistent with the stronger
signs of dust processing observed in these disks. For the case of one brown
dwarf, 2M04230607, we find the forsterite mass fraction to be a factor of ~3
higher in the outer disk compared to the inner disk region. Simple large-scale
radial mixing cannot account for this gradient in the dust chemical
composition, and some local crystalline formation mechanism may be effective in
this disk. The relatively high abundance of crystalline silicates in the outer
cold regions of brown dwarf disks provides an interesting analogy to comets. In
this context, we have discussed the applicability of the various mechanisms
that have been proposed for comets on the formation and the outward transport
of high-temperature material. We also present Chandra X-ray observations for
two Taurus brown dwarfs, 2M04414825 and CFHT-BD-Tau 9. We find 2M04414825,
which has a ~12% crystalline mass fraction, to be more than an order of
magnitude brighter in X-ray than CFHT-BD-Tau 9, which has a ~35% crystalline
mass fraction. Combining with previous X-ray data, we find the inner disk
crystalline mass fractions to be anti-correlated with the X-ray strength.Comment: Accepted in MNRA
Left-right loading dependence of shock response of (111)//(112) Cu bicrystals: Deformation and spallation
We investigate with molecular dynamics the dynamic response of Cu bicrystals with a special asymmetric grain boundary (GB), (111)//(112)〈110〉, and its dependence on the loading directions. Shock loading is applied along the GB normal either from the left or right to the GB. Due to the structure asymmetry, the bicrystals demonstrate overall strong left-right loading dependence of its shock response, including compression wave features, compression and tensile plasticity, damage characteristics (e.g., spall strength), effective wave speeds and structure changes, except that spallation remains dominated by the GB damage regardless of the loading directions. The presence or absence of transient microtwinning also depends on the loading directions
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Material dynamics under extreme conditions of pressure and strain rate
Solid state experiments at extreme pressures (10-100 GPa) and strain rates ({approx}10{sup 6}-10{sup 8}s{sup -1}) are being developed on high-energy laser facilities, and offer the possibility for exploring new regimes of materials science. These extreme solid-state conditions can be accessed with either shock loading or with a quasi-isentropic ramped pressure drive. Velocity interferometer measurements establish the high pressure conditions. Constitutive models for solid-state strength under these conditions are tested by comparing 2D continuum simulations with experiments measuring perturbation growth due to the Rayleigh-Taylor instability in solid-state samples. Lattice compression, phase, and temperature are deduced from extended x-ray absorption fine structure (EXAFS) measurements, from which the shock-induced {alpha}-{omega} phase transition in Ti and the {alpha}-{var_epsilon} phase transition in Fe are inferred to occur on sub-nanosec time scales. Time resolved lattice response and phase can also be measured with dynamic x-ray diffraction measurements, where the elastic-plastic (1D-3D) lattice relaxation in shocked Cu is shown to occur promptly (< 1 ns). Subsequent large-scale molecular dynamics (MD) simulations elucidate the microscopic dynamics that underlie the 3D lattice relaxation. Deformation mechanisms are identified by examining the residual microstructure in recovered samples. The slip-twinning threshold in single-crystal Cu shocked along the [001] direction is shown to occur at shock strengths of {approx}20 GPa, whereas the corresponding transition for Cu shocked along the [134] direction occurs at higher shock strengths. This slip-twinning threshold also depends on the stacking fault energy (SFE), being lower for low SFE materials. Designs have been developed for achieving much higher pressures, P > 1000 GPa, in the solid state on the National Ignition Facility (NIF) laser
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