1,859 research outputs found
High energy collision cascades in tungsten: dislocation loops structure and clustering scaling laws
Recent experiments on in-situ high-energy self-ion irradiation of tungsten
(W) show the occurrence of unusual cascade damage effects resulting from single
ion impacts, shedding light on the nature of radiation damage expected in the
tungsten components of a fusion reactor. In this paper, we investigate the
dynamics of defect production in 150 keV collision cascades in W at atomic
resolution, using molecular dynamics simulations and comparing predictions with
experimental observations. We show that cascades in W exhibit no subcascade
break-up even at high energies, producing a massive, unbroken molten area,
which facilitates the formation of large defect clusters. Simulations show
evidence of the formation of both 1/2 and interstitial-type
dislocation loops, as well as the occurrence of cascade collapse resulting in
vacancy-type dislocation loops, in excellent agreement with experimental
observations. The fractal nature of the cascades gives rise to a scale-less
power law type size distribution of defect clusters.Comment: 6 pages, 3 figure
Solar Fe abundance and magnetic fields - Towards a consistent reference metallicity
We investigate the impact on Fe abundance determination of including magnetic
flux in series of 3D radiation-MHD simulations of solar convection which we
used to synthesize spectral intensity profiles corresponding to disc centre. A
differential approach is used to quantify the changes in theoretical equivalent
width of a set of 28 iron spectral lines spanning a wide range in lambda,
excitation potential, oscillator strength, Land\'e factor, and formation
height. The lines were computed in LTE using the spectral synthesis code LILIA.
We used input magnetoconvection snapshots covering 50 minutes of solar
evolution and belonging to series having an average vertical magnetic flux
density of 0, 50, 100 and 200 G. For the relevant calculations we used the
Copenhagen Stagger code. The presence of magnetic fields causes both a direct
(Zeeman-broadening) effect on spectral lines with non-zero Land\'e factor and
an indirect effect on temperature-sensitive lines via a change in the
photospheric T-tau stratification. The corresponding correction in the
estimated atomic abundance ranges from a few hundredths of a dex up to |Delta
log(Fe)| ~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the IR. The
largest modification to previous solar abundance determinations based on
visible spectral lines is instead due to the indirect effect, i.e., the
line-weakening caused by a warmer stratification on an optical depth scale. Our
results indicate that the average solar iron abundance obtained when using
magnetoconvection models can be 0.03-0.11 dex higher than when using the
simpler HD convection approach. We demonstrate that accounting for magnetic
flux is important in state-of-the-art solar photospheric abundance
determinations based on 3D simulations.Comment: 12 pages, 7 figures, A&A in pres
A revised model of the active site of alternative oxidase
AbstractThe plant mitochondrial protein alternative oxidase catalyses dioxygen dependent ubiquinol oxidation to yield ubiquinone and water. A structure of this protein has previously been proposed based on an assumed structural homology to the di-iron carboxylate family of proteins. However, these authors suggested the protein has a very different topology than the known structures of di-iron carboxylate proteins. We have re-examined this model and based on comparison of recent sequences and structural data on di-iron carboxylate proteins we present a new model of the alternative oxidase which allows prediction of active site residues and a possible membrane binding motif
Molecular dynamics simulations of cascades in strained carbide inclusions embedded in alpha-iron
The effect of strain on the amount of point defects created in Fe and Cr carbide inclusions embedded in ferrite has been investigated. The spherical carbide inclusions consisted of either Fe3C or Cr23C6. Recoil energies from 100 eV to 3 keV and strains from -0.15 (compressive) to 0.01 (tensile) were used. The overall tendency is that the number of point defects - such as antisites, vacancy and interstitials - inside the carbide is lowered when the strain grows more negative (compressive). Outside the carbides, the number of defects is markedly higher for strongly compressive strains than for e.g. zero strain, especially at high energies. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.Peer reviewe
Magnetohydrodynamic turbulence in warped accretion discs
Warped, precessing accretion discs appear in a range of astrophysical
systems, for instance the X-ray binary Her X-1 and in the active nucleus of
NGC4258. In a warped accretion disc there are horizontal pressure gradients
that drive an epicyclic motion. We have studied the interaction of this
epicyclic motion with the magnetohydrodynamic turbulence in numerical
simulations. We find that the turbulent stress acting on the epicyclic motion
is comparable in size to the stress that drives the accretion, however an
important ingredient in the damping of the epicyclic motion is its parametric
decay into inertial waves.Comment: to appear in the proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
The response of a turbulent accretion disc to an imposed epicyclic shearing motion
We excite an epicyclic motion, whose amplitude depends on the vertical
position, , in a simulation of a turbulent accretion disc. An epicyclic
motion of this kind may be caused by a warping of the disc. By studying how the
epicyclic motion decays we can obtain information about the interaction between
the warp and the disc turbulence. A high amplitude epicyclic motion decays
first by exciting inertial waves through a parametric instability, but its
subsequent exponential damping may be reproduced by a turbulent viscosity. We
estimate the effective viscosity parameter, , pertaining to
such a vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
, pertaining to a horizontal (Keplerian) shear. We find that,
as is often assumed in theoretical studies, is approximately
equal to and both are much less than unity, for the field
strengths achieved in our local box calculations of turbulence. In view of the
smallness () of and we conclude
that for the timescale for diffusion
or damping of a warp is much shorter than the usual viscous timescale. Finally,
we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte
Molecular dynamics simulation of beryllium oxide irradiated by deuterium ions: sputtering and reflection
The sputtering and reflection properties of wurtzite beryllium oxide (BeO) subjected to deuterium (D) ions bombardment at 300 K with ion energy between 10 eV and 200 eV is studied by classical molecular dynamics. Cumulative irradiations of wurtzite BeO show a D concentration threshold above which an 'unphysical dramatic' sputtering is observed. From the cumulative irradiations, simulation cells with different D concentrations are used to run non-cumulative irradiations at different concentrations. Using a D concentration close to the experimentally determined saturation concentration (0.12 atomic fraction), the simulations are able to reproduce accurately the experimental sputtering yield of BeO materials. The processes driving the sputtering of beryllium (Be) and oxygen (O) atoms as molecules are subsequently determined. At low irradiation energy, between 10 eV and 80 eV, swift chemical sputtering (SCS) is dominant and produces mostly ODz molecules. At high energy, the sputtered molecules are mostly BexOy molecules (mainly BeO dimer). Four different processes are associated to the formation of such molecules: the physical sputtering of BeO dimer, the delayed SCS not involving D ions and the detachment-induced sputtering. The physical sputtering of BeO dimer can be delayed if the sputtering event implies two interactions with the incoming ion (first interaction in its way in the material, the other in its way out if it is backscattered). The detachment-induced sputtering is a characteristic feature of the 'dramatic' sputtering and is mainly observed when the concentration of D is close to the threshold leading to this sputtering regime.Peer reviewe
Probing 5f-state configurations in URu2Si2 with U L3-edge resonant x-ray emission spectroscopy
Resonant x-ray emission spectroscopy (RXES) was employed at the U L3
absorption edge and the La1 emission line to explore the 5f occupancy, nf, and
the degree of 5f orbital delocalization in the hidden order compound URu2Si2.
By comparing to suitable reference materials such as UF4, UCd11, and alpha-U,
we conclude that the 5f orbital in URu2Si2 is at least partially delocalized
with nf = 2.87 +/- 0.08, and does not change with temperature down to 10 K
within the estimated error. These results place further constraints on
theoretical explanations of the hidden order, especially those requiring a
localized f2 ground state.Comment: 11 pages,7 figure
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