4,038 research outputs found
Computation of free energy profiles with parallel adaptive dynamics
We propose a formulation of adaptive computation of free energy differences,
in the ABF or nonequilibrium metadynamics spirit, using conditional
distributions of samples of configurations which evolve in time. This allows to
present a truly unifying framework for these methods, and to prove convergence
results for certain classes of algorithms. From a numerical viewpoint, a
parallel implementation of these methods is very natural, the replicas
interacting through the reconstructed free energy. We show how to improve this
parallel implementation by resorting to some selection mechanism on the
replicas. This is illustrated by computations on a model system of
conformational changes.Comment: 4 pages, 1 Figur
Observation of single pion production if neutrino-like interactions without a charged lepton
Results of the 1967 CERN bubble chamber experiment have been re- analysed to find events consistent with the reactions: nu /sub mu /p to nu /sub mu /n pi /sup +/ nu /sub mu /p to nu /sub mu /p pi /sup 0/ nu /sub mu /n to nu /sub mu /n pi /sup 0/ nu /sub mu /n to nu /sub mu /p pi /sup -/ nu /sub mu /p to mu /sup -/p pi /sup +/
Cation distribution in manganese cobaltite spinels Co3−xMnxO4 (0 ≤ x ≤ 1) determined by thermal analysis
Thermogravimetric analysis was used in order to study the reduction in air of submicronic powders of Co3−x Mn x O4 spinels, with 0 ≤ x ≤ 1. For x = 0 (i.e. Co3O4), cation reduction occurred in a single step. It involved the CoIII ions at the octahedral sites, which were reduced to Co2+ on producing CoO. For 0 < x ≤ 1, the reduction occurred in two stages at increasing temperature with increasing amounts of manganese. The first step corresponded to the reduction of octahedral CoIII ions and the second was attributed to the reduction of octahedral Mn4+ ions to Mn3+. From the individual weight losses and the electrical neutrality of the lattice, the CoIII and Mn4+ ion concentrations were calculated. The distribution of cobalt and manganese ions present on each crystallographic site of the spinel was determined. In contrast to most previous studies that took into account either CoIII and Mn3+ or Co2+, CoIII and Mn4+ only, our thermal analysis study showed that Co2+/CoIII and Mn3+/Mn4+ pairs occupy the octahedral sites. These results were used to explain the resistivity measurements carried out on dense ceramics prepared from our powders sintered at low temperature (700–750 °C) in a Spark Plasma Sintering apparatus
The boundary Riemann solver coming from the real vanishing viscosity approximation
We study a family of initial boundary value problems associated to mixed
hyperbolic-parabolic systems:
v^{\epsilon} _t + A (v^{\epsilon}, \epsilon v^{\epsilon}_x ) v^{\epsilon}_x =
\epsilon B (v^{\epsilon} ) v^{\epsilon}_{xx}
The conservative case is, in particular, included in the previous
formulation.
We suppose that the solutions to these problems converge to a
unique limit. Also, it is assumed smallness of the total variation and other
technical hypotheses and it is provided a complete characterization of the
limit.
The most interesting points are the following two.
First, the boundary characteristic case is considered, i.e. one eigenvalue of
can be .
Second, we take into account the possibility that is not invertible. To
deal with this case, we take as hypotheses conditions that were introduced by
Kawashima and Shizuta relying on physically meaningful examples. We also
introduce a new condition of block linear degeneracy. We prove that, if it is
not satisfied, then pathological behaviours may occur.Comment: 84 pages, 6 figures. Text changes in Sections 1 and 3.2.3. Added
Section 3.1.2. Minor changes in other section
Laboratory validation of the dual-zone phase mask coronagraph in broadband light at the high-contrast imaging THD-testbed
Specific high contrast imaging instruments are mandatory to characterize
circumstellar disks and exoplanets around nearby stars. Coronagraphs are
commonly used in these facilities to reject the diffracted light of an observed
star and enable the direct imaging and spectroscopy of its circumstellar
environment. One important property of the coronagraph is to be able to work in
broadband light.
Among several proposed coronagraphs, the dual-zone phase mask coronagraph is
a promising solution for starlight rejection in broadband light. In this paper,
we perform the first validation of this concept in laboratory.
First, we recall the principle of the dual-zone phase mask coronagraph. Then,
we describe the high-contrast imaging THD testbed, the manufacturing of the
components and the quality-control procedures. Finally, we study the
sensitivity of our coronagraph to low-order aberrations (inner working angle
and defocus) and estimate its contrast performance. Our experimental broadband
light results are compared with numerical simulations to check agreement with
the performance predictions.
With the manufactured prototype and using a dark hole technique based on the
self-coherent camera, we obtain contrast levels down to between 5
and 17 in monochromatic light (640 nm). We also reach contrast
levels of between 7 and 17 in broadband
( nm, nm and %), which demonstrates the excellent chromatic performance of the dual-zone
phase mask coronagraph.
The performance reached by the dual-zone phase mask coronagraph is promising
for future high-contrast imaging instruments that aim at detecting and
spectrally characterizing old or light gaseous planets.Comment: 9 pages, 16 figure
Carbon nanotubes grown in situ by a novel catalytic method
Carbon nanotubes can be produced by the catalytic decomposition of hydrocarbons on small metal particles. However, nanotubes are generally produced together with non-tubular filaments and tubes coated by pyrolytic carbon. We propose a novel catalyst method for the in situ production, in a composite powder, of a huge amount
of single- and multiwalled carbon nanotubes, having a diameter between 1.5 and 15 nm and arranged in bundles up to 100 mm long. We anticipate that dense materials prepared from such composite powders could have interesting mechanical and physical properties
An aperture masking mode for the MICADO instrument
MICADO is a near-IR camera for the Europea ELT, featuring an extended field
(75" diameter) for imaging, and also spectrographic and high contrast imaging
capabilities. It has been chosen by ESO as one of the two first-light
instruments. Although it is ultimately aimed at being fed by the MCAO module
called MAORY, MICADO will come with an internal SCAO system that will be
complementary to it and will deliver a high performance on axis correction,
suitable for coronagraphic and pupil masking applications. The basis of the
pupil masking approach is to ensure the stability of the optical transfer
function, even in the case of residual errors after AO correction (due to non
common path errors and quasi-static aberrations). Preliminary designs of pupil
masks are presented. Trade-offs and technical choices, especially regarding
redundancy and pupil tracking, are explained.Comment: SPIE 2014 Proceeding -- Montrea
HE II Two Phase Flow in an Inclinable 22 m Long Line
In the line of previous work done at CEA Grenoble, large size experiments were performed with the support of CERN for the validation of the LHC two phase superfluid helium cooling scheme. In order to be as close as possible to the real configuration, a straight, inclinable 22 m long line of 40 mm I.D. was built. Very accurate measurements of temperatures and pressures obtained after in situ re-calibration and verified by independent sensors allowed us to validate our two-phase flow model. Although we focus on pressure losses and heat exchange results in relation to power injected, additional measurements such as quality, void fraction, and total mass flow rate enable a complete description of the two-phase flow. Experiments were carried out to cover the whole range of the future LHC He II two-phase flow heat exchanger pipe: slope between 0 and 2.8 %, temperature between 1.8 and 2 K, total mass flow rate up to 7.5 g/s. Results confirm the validity of choice for the LHC cooling scheme
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