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The small mission A-STAR (All-Sky Transient Astrophysics Reporter) aims to locate the
X-ray counterparts to ALIGO and other gravitational wave detector sources, to study the
poorly-understood low luminosity gamma-ray bursts, and to find a wide variety of transient
high-energy source types, A-STAR will survey the entire available sky twice per 24 hours.
The payload consists of a coded mask instrument, Owl, operating in the novel low energy
band 4−150 keV, and a sensitive wide-field focussing soft X-ray instrument, Lobster,
working over 0.15−5 keV. A-STAR will trigger on ~100 GRBs/yr, rapidly
distributing their locations.
With the development of applications involving high sensitivity ferromagnetic-ferroelectric laminates, a systematic analysis of the noise floor for magneto-electric (ME) laminated sensor has become crucial. We report and discuss the results of such an analysis on the noise floor of magnetostrictive-piezoelectric laminates in terms of the magnetic noise spectral density at room temperature. The noise floor of highly sensitive ME laminates with a JFET charge amplifier detection method has been studied. A good correlation was found between the theoretical and experimental noise curves within the measurement bandwidth. The dominating noise sources were found to include the dielectric loss noise, mechanical loss noise of the magneto-electric laminates and the noise sources of the charge amplifier. By using an appropriate low noise JFET charge amplifier, the noise contributions from the amplifier can be made negligible, enabling the measurement of the intrinsic noise of the ME laminates sensor. Thus, we have shown that at low frequencies, below the resonant frequency, the dielectric loss noise predominates with a one-per-root-frequency dependence whereas, around the resonance, the mechanical loss noise prevails over all other noise sources as expected from our theoretical analysis.
The present study is motivated by a need to produce stability modes to assist in the understanding and control of unsteady separated flows. The flow configuration is a NACA 0015 aerofoil with laminar leading-edge separation and turbulent recirculation. In previous water tunnel experiments, this flow configuration was measured in an unperturbed (uncontrolled) separated state, and a harmonically perturbed (controlled) reattached state. This study presents numerical data of the unperturbed case, and recovers stability modes to describe the evolution of perturbations in this environment. The unperturbed flow is numerically generated using large eddy simulation. Its temporal properties are quantified via a Fourier analysis of the velocity time history at selected points in space. The leading-edge shear layer instability is characterized by instantaneous vortex structures, and the bluff body shedding is illustrated by proper orthogonal decomposition modes. Statistical measures of the velocity field agree well with the water tunnel measurements. Finally a stability analysis is undertaken using a triple decomposition to distinguish between the time averaged field, the unsteady scales of motion, and a coherent wave (perturbation). This analysis identifies that perturbations in the region immediately downstream of the separated shear layer have the highest spatial growth rates. The associated frequency is of the order of the sub-harmonic of the shear layer instability.
The analysis of the instabilities in an unsteady turbulent flow is undertaken using a triple decomposition to distinguish between the time-averaged field, a coherent wave and the remaining turbulent scales of motion. The stability properties of the coherent scale are of interest. Previous studies have relied on prescribed constants to close the equations governing the evolution of the coherent wave. Here we propose an approach where the model constants are determined only from the statistical measures of the unperturbed velocity field. Specifically, a nonlinear eddy-viscosity model is used to close the equations, and is a generalisation of earlier linear eddy-viscosity closures. Unlike previous models the proposed approach does not assume the same dissipation rate for the time- and phase-averaged fields. The proposed approach is applied to a previously published turbulent channel flow, which was harmonically perturbed by two vibrating ribbons located near the channel walls. The response of the flow was recorded at several downstream stations by phase averaging the probe measurements at the same frequency as the forcing. The experimentally measured growth rates and velocity profiles, are compared to the eigenvalues and eigenvectors resulting from the stability analysis undertaken herein. The modes recovered from the solution of the eigenvalue problem, using the nonlinear eddy-viscosity model, are shown to capture the experimentally measured spatial decay rates and mode shapes of the coherent scale.
Recent advances in computer simulation at the atomic scale have made it possible to probe the structure and behaviour of the cores of dislocations in minerals. Such simulation offers the possibility to understand and predict the dislocation-mediated properties of minerals such as mechanisms of plastic deformation, pipe diffusion and crystal growth. In this review the three major methods available for the simulation of dislocation cores are described and compared. The methods are: (1) cluster-based models which combine continuum elastic theory of the extended crystal with an atomistic model of the core; (2) dipole models which seek to cancel the long-range elastic displacement caused by the dislocation by arranging for the simulation to contain several dislocations with zero net Burgers vector, thus allowing a fully periodic super-cell calculation; and (3) the Peierls-Nabarro approach which attempts to recast the problem so that it can be solved using only continuum-based methods, but parameterizes the model using results from atomic-scale calculations. The strengths of these methods are compared and illustrated by some of the recent studies of dislocations in mantle silicate minerals. Some of the unresolved problems in the field are discussed.
Hexagonal AlN thin films have been deposited by DC reactive magnetron
sputtering at room temperature. For a first set of samples, sputtered AlN
films were deposited on silicon Si (100) substrates. For a second set, AlN
films were deposited on 200 nm (002) oriented AlN epitaxial layer obtained
by Molecular Beam Epitaxy (MBE) on Si (111).
X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopy
(HRTEM) analysis of the synthesized films on Si (100) substrate have shown
an amorphous phase close to the interface followed by a nano-crystalline
layer exhibiting (100) and (002) orientations of the hexagonal AlN
crystalline phase. Finally a relatively well crystallised layer with a
single (002) orientation has been observed for the thickest films. This
improvement of crystalline quality with film thickness has been consistent
with a drastic decrease of the films stress from –1.2 GPa at 300 nm to no
stress around 800 nm and even 0.3 GPa tensile stress for 1.5 μm thick
film. This behaviour was different when epitaxial AlN was used as substrate.
In fact, we have observed thanks to HRTEM images and Selected Area Electron
Diffraction (SAED) patterns, that the AlN film deposited on such a substrate
exhibits the same crystalline quality and have the same orientation as the
AlN epitaxial layer during the first 500 nm of thickness. A further increase
of film thickness has caused a decrease on the crystalline quality. The
films became polycrystalline while preserving a (002) preferential
AlxGa1- xN/GaN high electron mobility transistor (HEMT) structures grown by ammonia-source molecular beam epitaxy (MBE) are focused-ion-beam implanted with 300 keV Gd-ions at room temperature. The two-dimensional electron gas (2DEG) of these HEMT structures is located 27 nm underneath the sample surface. At 4.2 K, current-voltage characteristics across implanted rectangles show that the structures remain conducting up to a Gd-dose of 1×1012 cm-2. Anomalous Hall effect (AHE) is observed at T = 4.2 K for structures implanted with Gd, whose dose is 3×1011 cm-2. Measurements of AHE in the wide temperature range from 2.4 K to 300 K show that the magnetic ordering temperature of these structures is around 100 K. Therefore, these Gd-implanted HEMT structures containing the still conducting 2DEG, which is now embedded in a ferromagnetic semiconductor, open the possibility to polarize the electron spins.
Depuis le début des années 90, EDF a fait des efforts importants en matière d’optimisation de la radioprotection des travailleurs dans les centres nucléaires de production d’électricité (CNPE) ce qui s’est traduit par une forte réduction de la dose individuelle (581 intervenants au dessus de 20 mSv en 1996 ; aucun au dessus de 20 mSv et seulement 17 au dessus de 16 mSv en 2006) et de la dose collective moyenne par tranche (réduction de plus d’un facteur deux en dix ans). EDF souhaite poursuivre ses efforts pour améliorer la radioprotection dans les centrales françaises. Une des voies d’amélioration est l’analyse du retour d’expérience de la pratique des centrales étrangères ayant de très bons résultats dosimétriques ou ayant connu une amélioration récente significative de la dosimétrie. Dans cette perspective, entre 2003 et 2006, le CEPN a organisé pour EDF huit visites d’intercomparaison de l’organisation et des pratiques de radioprotection dans des centrales nucléaires à l’étranger. Lors de chaque visite, une équipe de quatre personnes, composée de deux représentants d’EDF et de deux représentants du CEPN, se rend sur place pendant une semaine et rencontre des représentants de la plupart des services impliqués dans la mise en œuvre des thèmes retenus. Plusieurs bonnes pratiques de radioprotection ont été mises en avant lors de ces visites : une bonne intégration de la radioprotection dans le management de la centrale ; les efforts de réduction du terme source ; l’impact fondamental de la réduction des volumes de travail exposé par l’optimisation des programmes de maintenance et par des modifications entraînant une réduction de la nécessité de maintenance ; une radioprotection de proximité expérimentée ; de nouveaux outils au service de la radioprotection du 21e siècle, en particulier la télésurveillance et la télédosimétrie.
L’arrêté zonage du 15 mai 2006, relatif aux conditions de délimitation et de signalisation des zones surveillées et contrôlées, est entré en vigueur le 15 décembre 2006. Bien que les dispositions de zonage énoncées dans cet arrêté n'induisent pas de bouleversement, sa mise en œuvre impose une révision des pratiques. Ce document a pour objectif de présenter les évolutions réglementaires dans la méthodologie de zonage en s'appuyant sur des exemples dans différents domaines d’utilisation et d'aider ainsi à la mise en œuvre de cet arrêté.
A series of isothermal annealing experiments have been performed in the range 790–920°C under N2 flow in order to study the deuterium out-diffusion kinetics of Mg-doped GaN grown on sapphire under deuterated ammonia. The deuterium concentration was measured by SIMS analysis before and after each annealing step. The kinetics closely follow a first-order law. The activation energy related to the deuterium out-diffusion process is 3.1 eV. In addition, deuterium effusion measurements were performed measuring the molecular HD flux while the specimens were annealed in ultra high vacuum with a linear heating rate. In contrast to SIMS, this method detects the species that migrated out of the sample. Effusion peaks of the HD flux at 360 and 490°C are attributed to the fragmentation of adsorbed CHxDy complexes. The molecular HD flux starts increasing at 800°C which is the onset of the GaN decomposition and has its maximum at 920°C. This HD flux is accompanied by the desorption of H and D containing radicals and molecules desorbing above 900°C.
High power RF device performance
decreases as operation temperature increases (e.g. decreasing electron
mobility affects cut-off frequencies and degrades device reliability).
Therefore determination of device temperature is a key issue for device
topology optimisation. In this work the temperature variation of AlGaN/GaN
high-electron-mobility transistors grown either on silicon or sapphire
substrate under bias operation was measured by micro Raman scattering
spectroscopy. Temperature measurements up to power dissipation of 16 W for
4 mm development devices were carried out and a peak temperature of 650 K was
determined. The difference of thermal resistance for similar devices grown
on the two different substrates was assessed. The thermal resistances of
different device topologies were compared to optimise the component design.
We have studied the influence of a deuterium diffusion on the electrical characteristics of the 2D gas present in AlGaN/GaN heterostructures. The deuterium diffusion is performed by exposing the structures to a rf remote deuterium plasma. We find that both the sheet carrier concentration and the electron mobility decrease and that these effects are partly reversible under thermal annealing. These results suggest that deuterium behave as acceptors in the 2D gas region. The negatively charged deuterium act as additional scattering centers for electrons.
The high power RF device performance decreases as the operation
temperature increases (e.g. fall of electron mobility impacting
the cut-off frequencies and degradation of device reliability).
Therefore the determination of device temperature is a key issue
for device topology optimisation. In this work the temperature
variation of AlGaN/GaN high-electron-mobility transistors grown
either on silicon or sapphire substrates under bias operation was
measured by micro Raman scattering spectroscopy. The differences
in thermal resistance for similar devices grown on the two
different substrates were assessed. The thermal resistances of
different device topologies were compared in order to optimise
the component design. The temperature measurement across the gate
and along the component fingers were made to quantify the thermal
gradient of the device. Temperature measurement up to a power
dissipation of 16 W for a 4 mm development device was carried out
and the peak temperature of 650 K was determined.
The temporal dynamics of large-scale structures in a plane turbulent mixing layer
are studied through the development of a low-order dynamical system of ordinary
differential equations (ODEs). This model is derived by projecting Navier–Stokes
equations onto an empirical basis set from the proper orthogonal decomposition
(POD) using a Galerkin method. To obtain this low-dimensional set of equations, a
truncation is performed that only includes the first POD mode for selected streamwise/spanwise
(k1/k3) modes. The initial truncations
are for k3 = 0; however, once
these truncations are evaluated, non-zero spanwise wavenumbers are added. These
truncated systems of equations are then examined in the pseudo-Fourier space in
which they are solved and by reconstructing the velocity field. Two different methods
for closing the mean streamwise velocity are evaluated that show the importance
of introducing, into the low-order dynamical system, a term allowing feedback between
the turbulent and mean flows. The results of the numerical simulations show a
strongly periodic flow indicative of the spanwise vorticity. The simulated flow had the
correct energy distributions in the cross-stream direction. These models also indicated
that the events associated with the centre of the mixing layer lead the temporal
dynamics. For truncations involving both spanwise and streamwise wavenumbers,
the reconstructed velocity field exhibits the main spanwise and streamwise vortical
structures known to exist in this flow. The streamwise aligned vorticity is shown to
connect spanwise vortex tubes.
The mechanism by which dislocations move in the icosahedral quasicrystalline structure, i.e., glide or climb, is still an open question. In order to check whether pure dislocation glide occurs in this quasi-periodic structure, low temperature deformation tests have been performed under confining pressure conditions. These experimental techniques, which superimpose a shear stress on an isostatic component, enable the brittle-to-ductile transition temperature to be shifted to temperatures at which diffusion processes can be assumed to be negligible. Such techniques have been applied to deform plastically AlCuFe poly-quasicrystals at low and intermediate temperatures, using both gas and solid-confining media. Mechanical data as well as microstructural observations associated with this low temperature deformation range are reported. The first results provide new insights into the deformation mechanisms that control plasticity in the icosahedral quasicrystalline phase.
Large-scale structures in a plane turbulent mixing layer are studied through the use of
the proper orthogonal decomposition (POD). Extensive experimental measurements
are obtained in a turbulent plane mixing layer by means of two cross-wire rakes
aligned normal to the direction of the mean shear and perpendicular to the mean
flow direction. The measurements are acquired well into the asymptotic region. From
the measured velocities the two-point spectral tensor is calculated as a function of
separation in the cross-stream direction and spanwise and streamwise wavenumbers.
The continuity equation is then used for the calculation of the non-measured components
of the tensor. The POD is applied using the cross-spectral tensor as its kernel.
This decomposition yields an optimal basis set in the mean square sense. The energy
contained in the POD modes converges rapidly with the first mode being dominant
(49% of the turbulent kinetic energy). Examination of these modes shows that the
first mode contains evidence of both known flow organizations in the mixing layer, i.e.
quasi-two-dimensional spanwise structures and streamwise aligned vortices. Using the
shot-noise theory the dominant mode of the POD is transformed back into physical
space. This structure is also indicative of the known flow organizations.
After the first year of performance of SIGMA, more than 390 hours of data on the Galactic Centre region have been recorded. This survey establishes the extremely variable nature of this region. During a period of 6 months, 3 sources have dominated successively: 1E 1740.7-2942, GRS 1758-258, GX1 + 4. Concerning the two first sources, their spectral shape and their variability suggest to include them in the list of the black hole candidates.
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