1,000 research outputs found
Measurement of the electric fluctuation spectrum of magnetohydrodynamic turbulence
Magnetohydrodynamic (MHD) turbulence in the solar wind is observed to show
the spectral behavior of classical Kolmogorov fluid turbulence over an inertial
subrange and departures from this at short wavelengths, where energy should be
dissipated. Here we present the first measurements of the electric field
fluctuation spectrum over the inertial and dissipative wavenumber ranges in a
plasma. The inertial subrange is observed and
agrees strikingly with the magnetic fluctuation spectrum; the wave phase speed
in this regime is shown to be consistent with the Alfv\'en speed. At smaller
wavelengths the electric spectrum is softer and is consistent
with the expected dispersion relation of short-wavelength kinetic Alfv\'en
waves. Kinetic Alfv\'en waves damp on the solar wind ions and electrons and may
act to isotropize them. This effect may explain the fluid-like nature of the
solar wind.Comment: submitted; 4 pages + 3 figure
Thermal neutron induced (n,p) and (n,alpha) reactions on 37Ar
The 37Ar(n_th,alpha)34S and 37Ar(n_th,p)37Cl reactions were studied at the
high flux reactor of the ILL in Grenoble. For the 37Ar(n_th,alpha_0) and
37Ar(n_th,p) reaction cross sections, values of (1070+/-80)b and (37+/-4)b,
respectively, were obtained. Both values are about a factor 2 smaller than
results of older measurements. The observed suppression of the 37(n_th,alpha_1)
transition could be verified from theoretical considerations. Finally, evidence
was found for the two-step 37Ar(n_th,gamma-alpha) process.Comment: 11 pages, 5 figures, accepted for publication in Nuclear Physics
Spin-polarization-induced structural selectivity in Pd and Pt () compounds
Spin-polarization is known to lead to important {\it magnetic} and {\it
optical} effects in open-shell atoms and elemental solids, but has rarely been
implicated in controlling {\it structural} selectivity in compounds and alloys.
Here we show that spin-polarized electronic structure calculations are crucial
for predicting the correct crystal structures for Pd and Pt
compounds. Spin-polarization leads to (i) stabilization of the structure
over the structure in PtCr, PdCr, and PdMn, (ii) to the
stabilization of the structure over the structure in PdCo
and to (iii) ordering (rather than phase-separation) in PtCo and PdCr.
The results are analyzed in terms of first-principles local spin density
calculations.Comment: 4 pages, REVTEX, 3 eps figures, to appear in PR
Integration of formal fault analysis in ASSERT: Case studies and lessons learnt
International audienceThe ASSERT European Integrated Project (Automated proof-based System and Software Engineering for Real-Time systems; EC FP6, IST-004033) has investigated, elaborated and experimented advanced methods based on the AltaRica language and support tool OCAS for architecture and fault approach propagation description analysis, and integrated in the complete ASSERT process. The paper describes lessons learnt from three case studies: safety critical spacecraft, autonomous deep exploration spacecraft, and civil aircraft
Antideuterons as a Signature of Supersymmetric Dark Matter
Once the energy spectrum of the secondary component is well understood,
measurements of the antiproton cosmic-ray flux at the Earth will be a powerful
way to indirectly probe for the existence of supersymmetric relics in the
galactic halo. Unfortunately, it is still spoilt by considerable theoretical
uncertainties. As shown in this work, searches for low-energy antideuterons
appear in the mean time as a plausible alternative, worth being explored. Above
a few GeV/n, a dozen spallation antideuterons should be collected by the future
AMS experiment on board ISSA. For energies less than about 3 GeV/n, the
antideuteron spallation component becomes negligible and may be supplanted by a
potential supersymmetric signal. If a few low-energy antideuterons are
discovered, this should be seriously taken as a clue for the existence of
massive neutralinos in the Milky Way.Comment: 16 pages, 9 figure
Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection
conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta
(ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic
shear (strong guide field). Changing the simulation domain size, we find that
the ion response varies greatly. For reconnecting regions with scales
comparable to the ion Larmor radius, the ions do not respond to the
reconnection dynamics leading to ''electron-only'' reconnection with very large
quasi-steady reconnection rates. The transition to more traditional
''ion-coupled'' reconnection is gradual as the reconnection domain size
increases, with the ions becoming frozen-in in the exhaust when the magnetic
island width in the normal direction reaches many ion inertial lengths. During
this transition, the quasi-steady reconnection rate decreases until the ions
are fully coupled, ultimately reaching an asymptotic value. The scaling of the
ion outflow velocity with exhaust width during this electron-only to
ion-coupled transition is found to be consistent with a theoretical model of a
newly reconnected field line. In order to have a fully frozen-in ion exhaust
with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width
of at least several ion inertial lengths is needed. In turbulent systems with
reconnection occurring between magnetic bubbles associated with fluctuations,
using geometric arguments we estimate that fully ion-coupled reconnection
requires magnetic bubble length scales of at least several tens of ion inertial
lengths
SmartEx: a case study on user profiling and adaptation in exhibition booths
An investigation into user profiling and adaptation with exhibition booth as a case study is reported. First a review of the field of exhibitions and trade fairs and a summary introduction to adaptation and profiling are given. We then introduce three criteria for the evaluation of exhibition booth: effectiveness, efficiency and affect. Effectiveness is related the amount of information collected, efficiency is a measurement of the time taken to collect the information, and affect is the perception of the experience and the mood booth visitors have during and after their visit. We have selected these criteria to assess adaptive and profiled exhibition booths, we call smart exhibition (SmartEx). The assessment is performed with an experiment with three test conditions (non-profiled/non adaptive, profiled/non-adaptive and profiled adaptive presentations). Results of the experiment are presented along discussion. While there is significant improvements of effectiveness and efficiency between the two-first test conditions, the improvement is not significant for the last test condition, for reasons explained. As for the affect, the results show that it has an under-estimated importance in people minds and that it should be addressed more carefully
Dependence of direct neutron capture on nuclear-structure models
The prediction of cross sections for nuclei far off stability is crucial in
the field of nuclear astrophysics. We calculate direct neutron capture on the
even-even isotopes Sn and Pb with energy levels,
masses, and nuclear density distributions taken from different
nuclear-structure models. The utilized structure models are a
Hartree-Fock-Bogoliubov model, a relativistic mean field theory, and a
macroscopic-microscopic model based on the finite-range droplet model and a
folded-Yukawa single-particle potential. Due to the differences in the
resulting neutron separation and level energies, the investigated models yield
capture cross sections sometimes differing by orders of magnitude. This may
also lead to differences in the predicted astrophysical r-process paths.
Astrophysical implications are discussed.Comment: 25 pages including 12 figures, RevTeX, to appear in Phys. Rev.
Screened Coulomb interactions in metallic alloys: II Screening beyond the single-site and atomic sphere approximations
A quantitative description of the configurational part of the total energy of
metallic alloys with substantial atomic size difference cannot be achieved in
the atomic sphere approximation: It needs to be corrected at least for the
multipole moment interactions in the Madelung part of the one-electron
potential and energy. In the case of a random alloy such interactions can be
accounted for only by lifting the atomic sphere and single-site approximations,
in order to include the polarization due to local environment effects.
Nevertheless a simple parameterization of the screened Coulomb interactions for
the ordinary single-site methods, including the generalized perturbation
method, is still possible. We obtained such a parameterization for bulk and
surface NiPt alloys, which allows one to obtain quantitatively accurate
effective interactions in this system.Comment: 24 pages, 2 figure
ISAAC, a framework for integrated safety analysis of functional, geometrical and human aspects
International audienceThis paper aims at presenting methods and tools that are developed in the ISAAC project (Improvement of Safety Activities on Aeronautical Complex Systems, www.isaac-fp6.org), a European Community funded project, to support the safety assessment of complex embedded systems. The ISAAC methodology proposes to base as much of the safety analyses as is feasibly possible on simulable and formally verifiable system models that include fault models and can be shared both by safety and design engineers. On one hand, tools were developed to support safety assessment of Simulink, SCADE, Statemate, NuSMV and AltaRica models. On the other hand, formal models are coupled with additional models to address the problems of common cause analysis and human error analysis
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