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We characterized the radiation response in the visible domain of a new multimode graded-index (GI) phosphosilicate optical fiber (GIMMF), exposed to the harsh environment (pulses of 14-MeV neutrons, X-rays, and γ -rays) associated with laser experiments at the OMEGA facility. The growth of permanent radiation-induced attenuation (RIA) was measured in situ after a series of laser shots involving a large production of 14-MeV neutrons (yields > 10^14 n per shot). RIA linearly increases with accumulated neutron fluence without recovery between shots. The obtained results allow a precise evaluation of this GIMMF vulnerability when implemented as part of laser or plasma diagnostics. Our work also reveals the potential of this class of optical fiber to serve as a radiation monitor in the radiation-rich mixed environments of megajoule class laser facilities and to provide a very fast and online estimation of the accumulated deposited dose at various locations of their experimental halls. In our experimental test configuration at OMEGA, 14-MeV neutrons are estimated to contribute to about 55% of the total deposited dose on the fibers, and the other optical losses are related to X-ray and γ -ray contributions. Those measurements could be, for example, benchmarked to the radiation maps obtained by Monte Carlo simulation tools, potentially facilitating the evaluation of the aging of diagnostics, components, and systems as well as their maintenance operations.

International audience; HPC systems have experienced significant growth over the past years, with modern machines having hundreds of thousands of nodes. Message Passing Interface (MPI) is the de facto standard for distributed computing on these architectures. On the MPI critical path, the message-matching process is one of the most time-consuming operations. In this process, searching for a specific request in a message queue represents a significant part of the communication latency. So far, no miracle algorithm performs well in all cases. This paper explores potential matching specializations thanks to hints introduced in the latest MPI 4.0 standard. We propose a hash-table-based algorithm that performs constant time message-matching for no wildcard requests. This approach is suitable for intensive point-to-point communication phases in many applications (more than 50% of CORAL benchmarks). We demonstrate that our approach can improve the overall execution time of real HPC applications by up to 25%. Also, we analyze the limitations of our method and propose a strategy for identifying the most suitable algorithm for a given application. Indeed, we apply machine learning techniques for classifying applications depending on their message pattern characteristics.

In this paper, we propose an adaptive mesh refinement method for 2D multi-material compressible nonviscous flows in semi-Lagrangian coordinates. The mesh adaptation procedure is local and relies on discrete metric field evaluation. The remapping method is second-order accurate and we prove its stability. We propose a multi-material treatment using two ingredients: the local remeshing is performed in a way to reduce as much as possible mixture creation and an interface reconstruction method is used to avoid material diffusion in mixing cells. The obtained method is almost Lagrangian and can be implemented in a parallel framework. We provide some numerical tests which attest the validity of the method and its robustness.

International audience; Finding the ground-state configuration of point defects in semiconductors is in general a high-dimensional optimization problem. However, their adiabatic potential energy surfaces (APES) can be described with few effective coordinates by exploiting symmetry arguments within the Jahn-Teller (JT) theoretical framework. In this paper, we propose a general framework, that combines group theory and ab initio calculations, to build a full picture of common defects in diamond lattices (vacancies and substitutional impurities) as instances of the general T ⊗ (e + t 2) APES. Starting from the original tetrahedral symmetry, we consider all possible JT symmetries and we numerically investigate the shape of the APES by targeting the minimum energy path between equivalent JT distortions taking as examples the vacancy (V) and the metallic impurities Pd, Pt, and Au in crystalline silicon.

The ultrafast electron energy transport is investigated in laser-heated warm dense copper in a high flux regime (2.5±0.7×10^{13} W/cm^{2} absorbed). The dynamics of the electron temperature is retrieved from femtosecond time-resolved x-ray absorption near-edge spectroscopy near the Cu L3 edge. A characteristic time of ∼1 ps is observed for the increase in the average temperature in a 100 nm thick sample. Data are well reproduced by two-temperature hydrodynamic simulations, which support energy transport dominated by thermal conduction rather than ballistic electrons.

We detail results of an experiment performed at the Ligne d'Intégration Laser facility aimed at studying supersonic and diffusive radiation front propagation in low-density SiO2 aerogel (20 and 40 mg/cm3) enclosed in a gold tube, driven by thermal emission from a laser-heated spherical gold cavity. The evolution of the front is studied continuously by measuring its self-emission with a 1D (one-dimensional) time-resolved soft x-ray imager. Measurement is performed along (through a 200- μm-wide observation slit) and at the exit of the tube giving access to the dynamics and the curvature of the front. Experimental results are then compared successfully to results from the 3D (three-dimensional) radiation hydrodynamics code TROLL, which shows that if continuous tracking of the front position is accessible with this experimental scheme, measurement of its maximum radiation temperature is on the contrary affected by radiation closure of the observation slit. 3D simulations also indicate that this effect can even be worsened if one includes pointing errors of the x-ray imager. Radiation temperature along the tube was then inferred by combining results from the imager to a wall shock breakout time measurement using a velocity interferometer system for any reflector and results from a broadband x-ray spectrometer used to determine the temperature at the exit of the tube. A decrease in the radiation temperature along the tube is observed, the decrease being more important for the higher SiO2 aerogel density.

Two approaches are proposed to simulate tsunamis generated by granular landslides: a depth-averaged model, AVALANCHE, and laminar Navier–Stokes simulations using the OpenFOAM model. Both models are validated against two 2D benchmarks, a subaerial and a submerged one involving a triangle initial slide shape. In both models, the landslide is defined as a viscous fluid flowing downslope. Except in the first instants, both models can reproduce either the landslide behavior or the generated first water waves but cannot reproduce simultaneously both the landslide and the water surface. There is an overlap in the optimal viscosity range between both models. Sensitivity studies are carried out by varying the slope angle and the landslide submergence for the subaerial benchmark. The largest waves are obtained for initial landslide position close to the free surface. The height of the generated waves increases linearly with the slope angle and the landslide Reynolds number in the depth-averaged model. The relationships are more complex in the Navier–Stokes model. For low slide Reynolds numbers and with an initial slide close to the free surface, both models produce similar waves. Nine additional cases are performed with a k − e turbulence closure model and varying the submergence and the slope angle.

The mantle transition zone (MTZ) of the Earth lies between 410 and ∼1000 km in depth and has a key role in mantle convection processes. In particular, the discontinuity at 660 km and its associated endothermic mineralogical transformation can slow or inhibit the passage of matter between the upper and lower mantle. The MTZ thus acts as a boundary layer within the mantle. The depth variations of radial and azimuthal seismic anisotropies enable the detection of boundary layers within the mantle. However, the 3D imaging is difficult due to the lack of sensitivity of surface waves of fundamental modes, and the poor global coverage of this depth range by body-wave data. We present a new 3D general anisotropy model (both radial and azimuthal anisotropies) of the mantle down to 1200 km in depth using surface-wave overtone datasets. We find that there is little seismic anisotropy in most of the MTZ, except below subduction zones around the Pacific Ocean and, more surprisingly, in a large area beneath eastern Eurasia where the Pacific subducting plate is stagnant. Seismic anisotropy is usually associated with intense deformation processes but also possibly to water transportation or to fine layering. This significant anisotropy in this part of MTZ might reveal a large water ‘reservoir’ associated with hydrous minerals or a strong stratification. It reflects a complex history beneath central Asia, where the Tethys, Izanagi and Pacific plates appear to have strongly interacted during the last 100 My, having subducted in orthogonal directions under the Asian continent, with the Tethys plate descending into the lower mantle, and the Izanagi plate remaining stagnant in the MTZ. The Asian continent is the only region in the world where subducting slabs originating from different plates can interact. This unique slab distribution might explain why some plates descend while others remain in the lower transition zone.

International audience; As graphite is an important material for several applications, the evaluation of its properties at very high temperatures is of major importance. Conducting studies at temperatures >2800 K is, however, very challenging, and the amount of available data in the literature is, therefore, very limited. This paper presents a methodology that compares experimental data with simulation results following a progressive increase of the temperature range investigated. The study was conducted on a commercial polycrystalline graphite produced by sintering (EDM-3 from POCO) under high power continuous laser heating (kW ytterbium at a wavelength of 1080 nm). Experiments were done inside a vacuum chamber equipped with pyrometers and cameras allowing fine monitoring of the temperature of the samples. A 3D numerical model has been developed based on the finite element method to analyze the experimental results. The evolution of the thermal and optical properties of EDM-3 with temperature is required for correctly modeling laser/material interactions. By running a sequential comparison of calculation with a dedicated experiment, we achieve better knowledge of such properties. From this study, we obtain estimations of the evolution of thermal conductivity, emissivity, and evaporation rate up to 3800 K of amorphous graphite.

International audience

Novel properties associated with nanothermites have attracted great interest for several applications, including lead-free primers and igniters. However, the prediction of quantitative structure-energetic performance relationships is still challenging. This study investigates machine learning methods as tools to surrogate complex physical models to design novel nanothermites with optimized burning rates chosen for energetic performance. The study focuses on Al/CuO nanolaminates, for which nine supervised regressors commonly used in ML applied to materials science are investigated. For each, an ML model is built using a database containing a set of 2700 Al/CuO nanolaminate systems, specifically generated for this study. We demonstrate the superiority of the multilayer perceptron algorithm to surrogate conventional physical-based models and predict the Al/CuO nanolaminate microstructure-burn rate relationship with good efficiency: the burn rate is estimated with less than 1% error (0.07 m·s

International audience; The inclusion of sophisticated density-dependent electronic stopping and electron-phonon coupling calculated with first-principles methods into molecular dynamics simulations of collision cascades has recently become possible thanks to the development of the so-called EPH (for Electron-PHonon) model. This work aims at employing the EPH model in molecular dynamics simulations of collision cascades in Si. In this context, the electronic stopping power is investigated in Si at low energies with Ehrenfest Dynamics calculations. Also, the parametrization of the EPH model for Si, from firstprinciples Ehrenfest Dynamics simulations to actual molecular dynamics simulations of collision cascades, is performed and detailed. We demonstrate that the EPH model is able to reproduce very closely the density-dependent features of the energy lost to electrons obtained with ab initio calculations. Molecular dynamics collision cascade simulations results obtained in Si using the EPH model and the simpler but widely employed Two Temperature Model (TTM) are compared, showing important discrepancies in the collision cascades results obtained depending on the model employed.

International audience; This work considers the challenging problem of identifying the statistical properties of random fields from indirect observations. To this end, a Bayesian approach is introduced, whose key step is the nonparametric approximation of the likelihood function from limited information. When the likelihood function is based on the evaluation of an expensive computer code, this work also proposes a method to select iteratively new design points to reduce the uncertainties on the results that are due to the approximation of the likelihood. Two applications are finally presented to illustrate the efficiency of the proposed procedure: a first one based on analytic data, and a second one dealing with the identification of the random elasticity field of an heterogeneous microstructure.

To amortize the cost of MPI collective operations, nonblocking collectives have been proposed so as to allow communications to be overlapped with computation. Unfortunately, collective communications are more CPU-hungry than point-to-point communications and running them in a communication thread on a dedicated CPU core makes them slow. On the other hand, running collective communications on the application cores leads to no overlap. In this article, we propose placement algorithms for progress threads that do not degrade performance when running on cores dedicated to communications to get communication/computation overlap. We first show that even simple collective operations, such as those based on a chain topology, are not straightforward to make progress in background on a dedicated core. Then, we propose an algorithm for tree-based collective operations that splits the tree between communication cores and application cores. To get the best of both worlds, the algorithm runs the short but heavy part of the tree on application cores, and the long but narrow part of the tree on one or several communication cores, so as to get a trade-off between overlap and absolute performance. We provide a model to study and predict its behavior and to tune its parameters. We implemented both algorithms in the multiprocessor computing framework, which is a thread-based MPI implementation. We have run benchmarks on manycore processors such as the KNL and Skylake and get good results for both performance and overlap.

International audience; This study investigates the leakage currents as well as the leakage current random telegraph signals sources in sense node floating diffusions and their consequences on imaging performances specifically after exposure to high energy particle radiation. Atomic displacement damage and ionization effects are separately studied thanks to neutron and X-ray irradiations. Proton irradiations have been performed to simultaneously study displacement damage dose and total ionizing dose effects while being more representative of the space environment. The studied displacement damage doses range from 500 TeV.g-1 to 40 GeV.g-1 and the total ionizing dose ranges from 24 krad(SiO2) to 72 krad(SiO2). High magnitude electric field effects, such as transfer gate induced leakage current, are investigated to further understand the phenomena involved in floating diffusions while giving new insights on the electric field enhancement of the charge generation mechanisms. This work shows that floating diffusions are very sensitive to ionizing radiation because of the presence of depleted Si/SiO2 interfaces with high magnitude electric fields around the junction. On the other hand, displacement damage in the floating diffusions is a major source of high amplitude leakage current random telegraph signals and leakage current non-uniformity. Such radiation-induced degradations can prevent the use of CMOS image sensor with a long floating diffusion retention time (e.g. global shutter operating mode or burst imagers) in radiation environments.

International audience; The hot deformation behavior and associated microstructural evolution of a coarse-grainedNb-bearing austenitic stainless steel (316Nb) has been investigated by the means of torsion testsat high temperature [1223 K to 1423 K (950 C to 1150 C)] followed by microstructuralcharacterization. The starting microstructure was varied by applying prior annealing. Except forstrains below 10 pct, the resistance to viscoplastic flow is not sensitive to the startingmicrostructure. On the other hand, prior annealing at higher temperatures increases theresistance to incipient viscoplastic flow and strongly impedes dynamic and post-dynamicrecrystallization by delaying the grain boundary bulging phenomenon. It also affects the staticrecrystallization behavior during further annealing. The influence of the amount of availableniobium atoms, in particular close to grain boundaries, is discussed.

International audience; This study presents experiments and numerical simulations of a beam with non-ideal boundary conditions and subjected to two broadband correlated or uncorrelated random excitations. Modeling of a beam with non-ideal boundary conditions and additional static pretension, as well as the modeling of multipoint correlated or uncorrelated random excitations, are developed. Based on numerical simulations via an extension of the Harmonic Balance Method, comparisons between experiments and numerical simulations are performed for various broadband bi-point correlated random excitations. In order to achieve such an objective, experiments are performed for four types of correlation between the two broadband random excitations: correlated excitations in phase, opposite phase and quadrature phase, and uncorrelated excitations. Experimental results demonstrate that the different configurations of broadband random excitations drastically affect, not only the amplification or attenuation of some symmetric or antisymmetric modes of the beam in the vicinity of their primary resonance, but also the appearance of harmonics and a combination of harmonic components. Good correlations are observed between experiments and numerical simulations, thus validating the proposed modeling and the computational strategy for the prediction of the nonlinear vibrational phenomena of the beam system subjected to two correlated or uncorrelated broadband random excitations. To be noted that the experimental data set including the input and output measurements of the four configurations are provided as supplementary files on Talik et al. [1].

The reaction of lithium hydride (LiH) powder with pure water vapor (H2O and D2O) was studied by thermogravimetry and in situ infrared spectroscopy at 298 K over a large pressure range. The mean particle size of LiH is around 27 μm. At very low pressure, the hydrolysis starts with the formation of lithium oxide (Li2O). Then, both Li2O and lithium hydroxide (LiOH) are formed on increasing pressure, thus, creating a Li2O/LiOH bilayer. The reaction takes place through the consumption of LiH and the formation of Li2O at the LiH/Li2O interface and through the consumption of Li2O and the formation of LiOH at the Li2O/LiOH interface. Above 10 hPa, only the monohydrate LiOH·H2O is formed. This hydration reaction of LiOH into LiOH·H2O occurs at a lower pressure (8 hPa) after the first hydration-dehydration cycle. The hydrolysis mechanism proposed in this paper suggests that the diffusion of ionic species across the intermediate Li2O layer is the rate limiting step of the reaction.

International audience; The spatial localization of whistler mode waves, determined by their generation process, propagation, and damping, is important for assessing the efficiency of wave-particle interactions and the dynamics of the radiation belts. We use 11 years of multipoint wave measurements in 2007-2017 from five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft covering L = 2-10 at low magnetic latitudes over all magnetic local times (MLTs) to characterize both the instantaneous spatial extent of chorus and hiss active regions and their wave amplitude distribution in the vicinity of their respective generation regions, making use of the time domain correlation technique. The correlation of lower band chorus wave power dynamics from two spacecraft measurements remains significant up to 250-800 km transverse to the background magnetic field, in agreement with the typical scale of a chorus element, with a shorter correlation length for higher peak wave power. Additionally, a coordinated analysis of average chorus amplitudes at separate locations shows that the radial extent of the chorus active region varies from 0.2 RE on the nightside to 1 RE on the dayside, with an azimuthal width of 1 MLT. The wave power correlation scale of hiss has no fine structure and is generally larger (1,500-3,000 km); its correlation remains nonnegligible up to 1.5 RE. Such observations are consistent with chorus generation by localized electron injections and subsequent hiss amplification at lower L shells from seed chorus waves having propagated and spread over a wider region inside the plasmasphere. The connection between hiss and chorus is further explored statistically through the correlation between their respective wave power variations, suggesting a significant link between them.

International audience; An original 3D stochastic model, based on the truncated plurigaussian random fields, has been adapted to simulate the complex microstructure of SOC electrodes. The representativeness of the virtual microstructures has been checked on several synchrotron X-ray and FIB-SEM tomographic reconstructions obtained on typical LSCF, LSC and Ni-YSZ electrodes. The validation step has been carried out by comparing numbers of electrode morphological properties as well as the phase effective diffusivities. This analysis has shown that the synthetic media mimic accurately the complex microstructure of typical SOC electrodes. The model capability to simulate different types of promising electrode architectures has also been investigated. It has been shown that the model is able to generate virtual electrode prepared by infiltration resulting in a uniform and continuous thin layer covering a scaffold. With a local thresholding depending on the position, continuous graded electrodes can be also produced. Finally, the model offers the possibility to introduce different correlation lengths for each phase in order to control the local topology of the interfaces. All these cases illustrate the model flexibility to generate various SOC microstructures. This validated and flexible model can be used for further numerical microstructural optimizations to improve the SOC performances.

Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m−1 K−1 for EB-PVD YSZ coatings to about 0.7 W m−1 K−1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni3Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.

International audience; For the first time, absolute densities of atomic nitrogen in its ground state (N$^4$S) have been measured in hot dry and humid air plasma columns under post-discharge regime. The determination of space-resolved absolute densities leads to obtain the dissociation degrees of molecular nitrogen in the plasma. The hot plasmas are generated inside an upstream gas-conditioning cell at 600 mbar when the air gas flow is directly injected at 10 slm in a microwave resonant cavity (2.45 GHz, 1 kW) placed in the downstream side. Density measurements based on laser induced fluorescence spectroscopy with two-photon excitation (TALIF), are more particularly performed along the radial and axial positions of the plasma column. Calibration of TALIF signals is performed in situ (i.e. in the same gas-conditioning cell but without plasma) using an air gas mixture containing krypton. Optical emission spectroscopy is considered to estimate the rotational gas temperature by adding a small amount of H$_2$ in dry air to better detect OH (A-X) spectra. The rotational temperatures in humid air plasma column (50% of humidity) are larger than those of dry air plasma column by practically 30% near the nozzle of resonant cavity on the axis of the plasma column. This is partly due to attachment heating processes initiated by water vapor. A maximum of the measured absolute nitrogen density is also observed near the nozzle which is also larger for humid air plasma column. The obtained dissociation degrees of molecular nitrogen in both dry and humid air plasma along the air plasma column are lower than the cases where only thermodynamic equilibrium is assumed. This is characteristic of the absence of chemical and energetic equilibria not yet reached in the air plasma column dominated by recombination processes.

Optical fibers hold promise for accurate dosimetry in small field proton therapy due to their superior spatial resolution and the lack of significant Cerenkov contamination in proton beams. One known drawback for most scintillation detectors is signal quenching in areas of high linear energy transfer, as is the case in the Bragg peak region of a proton beam. In this study, we investigated the potential of innovative optical fiber bulk materials using the sol-gel technique for dosimetry in proton therapy. This type of glass is made of amorphous silica (SiO$${}_{2}$$ 2 ) and is doped with Gd$${}^{3+}$$ 3 + ions and possesses very interesting light emission properties with a luminescence band around 314 nm when exposed to protons. The fibers were manufactured at the University of Lille and tested at the TRIUMF Proton Therapy facility with 8.2–62.9 MeV protons and 2–6 nA of extracted beam current. Dose-rate dependence and quenching were measured and compared to other silica-based fibers also made by sol-gel techniques and doped with Ce$${}^{3+}$$ 3 + and Cu$${}^{+}$$ + . The three fibers present strong luminescence in the UV (Gd) or visible (Cu,Ce) under irradiation, with the emission intensities related directly to the proton flux. In addition, the 0.5 mm diameter Gd$${}^{3+}$$ 3 + -doped fiber shows superior resolution of the Bragg peak, indicating significantly reduced quenching in comparison to the Ce$${}^{3+}$$ 3 + and Cu$${}^{+}$$ + fibers with a Birks’ constant, k$${}_{B}$$ B , of (0.0162 $$\pm $$ ± 0.0003) cm/MeV in comparison to (0.0333 $$\pm $$ ± 0.0006) cm/MeV and (0.0352 $$\pm $$ ± 0.0003) cm/MeV, respectively. To our knowledge, this is the first report of such an interesting k$${}_{B}$$ B for a silica-based optical fiber material, showing clearly that this fiber presents lower quenching than common plastic scintillators. This result demonstrates the high potential of this inorganic fiber material for proton therapy dosimetry.

International audience; An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Yang, Qian, et al., We present new Gemini/GMOS optical spectroscopy of 16 extreme variability quasars (EVQs) that dimmed by more than 1.5 mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey epochs (separated by a few years in the quasar rest frame). These EVQs are selected from quasars in the SDSS Stripe 82 region, covering a redshift range of 0.5 < z < 2.1. Nearly half of these EVQs brightened significantly (by more than 0.5 mag in the g band) in a few years after reaching their previous faintest state, and some EVQs showed rapid (non-blazar) variations of greater than 1–2 mag on time-scales of only months. To increase sample statistics, we use a supplemental sample of 33 EVQs with multi-epoch spectra from SDSS that cover the broad Mg II λ2798 line. Leveraging on the large dynamic range in continuum variability between the multi-epoch spectra, we explore the associated variations in the broad Mg II line, whose variability properties have not been well studied before. The broad Mg II flux varies in the same direction as the continuum flux, albeit with a smaller amplitude, which indicates at least some portion of Mg II is reverberating to continuum changes. However, the full width at half-maximum (FWHM) of Mg II does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg II to estimate the black hole mass with single epoch spectra therefore introduces a luminosity-dependent bias., The DES data management system is supported by the National Science Foundation under Grant Numbers AST-1138766 and AST-1536171. The DES participants from Spanish institutions are partially supported by MINECO under grants AYA2015- 71825, ESP2015-66861, FPA2015-68048, SEV-2016-0588, SEV2016-0597, and MDM-2015-0509, some of which include ERDF funds from the European Union. IFAE is partially funded by the CERCA program of the Generalitat de Catalunya. Research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013) including ERC grant agreements 240672, 291329, and 306478. We acknowledge support from the Brazilian Instituto Nacional de Cienciae Tecnologia (INCT) e-Universe (CNPq grant ˆ 465376/2014-2).

International audience; We study the dynamics of radiation belt electrons during a 10-day quiet period perturbed by substorm activity and preceding a high-speed stream (HSS), aiming at a global description of the radiation belts in L-shell, L in [2, 6], and energy [0.1, 10] MeV. We combine Van Allen Probes observations and Fokker-Planck numerical simulations of pitch-angle diffusion. The Fokker-Planck model uses event-driven pitch angle diffusion coefficients from whistler-mode waves, built from the wave properties and the ambient plasma density measurements from the Van Allen Probes. We first find this event has some similar characteristics to regular quiet times previously studied; a widely extended plasmasphere within which we observe strong and varying whistler-mode waves. These ambient conditions lead to strong pitch-angle scattering, which contributes to the creation of a wide slot region as well as a significant decay of the outer radiation belts, which are observed and qualitatively well simulated. In addition, we find the substorm activity causes short duration (within ± 4h) decay of the plasma density and a lowering amplitude of the whistler-mode waves within the plasmasphere, both causing opposite effects in terms of pitch angle diffusion. This leads to a diminution of pitch-angle diffusion at the time of the main substorm activity. Conversely, whistler-mode waves become enhanced in the time periods between the substorm injections. All effects cumulated, we find an enhancement of pitch angle diffusion by whistler-mode waves above L~4.7 during the 10-day period. This directly relates to the combination of quietness and substorm activity which allows pitch angle diffusing of up to 1 MeV electrons in the outer belt. Relativistic electrons of 1–2 MeV remain trapped in the outer belt, from L~4.7 to L~5.2, forming, in both the observations and the simulations, a distinct pocket of remnant electrons.

International audience; La Soufrière of Guadeloupe is an active volcano of Lesser Antilles that is closely monitored due to a high eruptive hazard potential. Since 1992 it exhibits a medium-level but sustained background hydrothermal activity with low-energy and shallow seismicity, hot springs temperature increase and high flux acidic gas fumaroles at the summit. The problem of estimating the heat balance and quantifying the evolution of hydrothermal activity has become a key challenge for surveillance. This work is the first attempt of a global mapping and quantification of La Soufrière thermal activity performed in February 2010 using aerial thermal infrared imagery. After instrument calibration and data processing, we present a global map of thermal anomalies allowing to spot the main active sites: the summit area (including the fumaroles of Tarissan Pit and South Crater), the Ty Fault fumarolic zone, and the hot springs located at the vicinity of the dome. In a second step, we deduce the mass and the energy fluxes released by the volcano. In particular, we propose a simple model of energy balance to estimate the mass flux of the summit fumaroles from their brightness temperature and size. In February 2010, Tarissan Pit had a 22.8 ± 8.1 kg s -1 flux (1970 ± 704 tons day -1), while South Crater vents had a total of 19.5 ± 4.0 kg s -1 (1687 ± 348 tons day -1). Once converted into energy flux, summit fumaroles represent 98% of the 106 ± 30 MW released by the volcano, the 2% remaining being split between the hot springs and the thermal anomalies at the summit and at the Ty Fault fumarolic zone. These values are in the high range of the previous estimations, highlighting the short-term variability of the expelled fluxes. Such a heat flux requires the cooling of 1500 m 3 of magma per day, in good agreement with previous geochemical studies.

International audience; In this paper we review recent spacecraft observations of oblique whistler-mode waves in the Earth’s inner magnetosphere as well as the various consequences of the presence of such waves for electron scattering and acceleration. In particular, we survey the statistics of occurrences and intensity of oblique chorus waves in the region of the outer radiation belt, comprised between the plasmapause and geostationary orbit, and discuss how their actual distribution may be explained by a combination of linear and non-linear generation, propagation, and damping processes. We further examine how such oblique wave populations can be included into both quasi-linear diffusion models and fully nonlinear models of wave-particle interaction. On this basis, we demonstrate that varying amounts of oblique waves can significantly change the rates of particle scattering, acceleration, and precipitation into the atmosphere during quiet times as well as in the course of a storm. Finally, we discuss possible generation mechanisms for such oblique waves in the radiation belts. We demonstrate that oblique whistler-mode chorus waves can be considered as an important ingredient of the radiation belt system and can play a key role in many aspects of wave-particle resonant interactions.

The full tensorial surface stress of an interface between a face-centered cubic crystal (copper) and an isotropic liquid (methane) is computed for two crystal orientations {100} and {110} using molecular dynamics simulations. The bulk crystal orientation {100} is symmetric, whereas the {110} orientation is not. Finite size effects, which can be important in the case of an interface between an isotropic solid and a liquid, are studied in detail for the two crystal orientations. We first show that the symmetry of the surface stress tensor is that of the bulk crystal orientation. In the case of the asymmetric crystal orientation {110}, the relative difference between the components of the surface stress is substantial (∼50%). Finally, we show that finite size effects persist to much larger sizes in the case of the {100} orientation compared to the case of the {110} interface, for instance, through an artificial breakdown of the symmetry of the surface stress tensor.

Ceramic materials are commonly used to design multi-layer armour systems thanks to their favourable physical and mechanical properties. However, during an impact event, fragmentation of the ceramic plate inevitably occurs due to its inherent brittleness under tensile loading. Consequently, an accurate model of the fragmentation process is necessary in order to achieve an optimum design for a desired armour configuration. In this work, shockless spalling tests have been performed on two silicon carbide grades at strain rates ranging from 103to 104 s−1using a high-pulsed power generator. These spalling tests characterize the tensile strength strain rate sensitivity of each ceramic grade. The microstructural properties of the ceramics appear to play an important role on the strain rate sensitivity and on the dynamic tensile strength. Moreover, this experimental configuration allows for recovering damaged, but unbroken specimens, giving unique insight on the fragmentation process initiated in the ceramics. All the collected data have been compared with corresponding results of numerical simulations performed using the Denoual–Forquin–Hild anisotropic damage model. Good agreement is observed between numerical simulations and experimental data in terms of free surface velocity, size and location of the damaged zones along with crack density in these damaged zones.This article is part of the themed issue ‘Experimental testing and modelling of brittle materials at high strain rates’.

This work introduces original explicit solutions for the elastic fields radiated by non-uniformly moving, straight, screw or edge dislocations in an isotropic medium, in the form of time-integral representations in which acceleration-dependent contributions are explicitly separated out. These solutions are obtained by applying an isotropic regularization procedure to distributional expressions of the elastodynamic fields built on the Green tensor of the Navier equation. The obtained regularized field expressions are singularity-free, and depend on the dislocation density rather than on the plastic eigenstrain. They cover non-uniform motion at arbitrary speeds, including faster-than-wave ones. A numerical method of computation is discussed, that rests on discretizing motion along an arbitrary path in the plane transverse to the dislocation, into a succession of time intervals of constant velocity vector over which time-integrated contributions can be obtained in closed form. As a simple illustration, it is applied to the elastodynamic equivalent of the Tamm problem, where fields induced by a dislocation accelerated from rest beyond the longitudinal wave speed, and thereafter put to rest again, are computed. As expected, the proposed expressions produce Mach cones, the dynamic build-up and decay of which is illustrated by means of full-field calculations., Comment: 36 pages, 6 figures. v2: metadata amended. v3: a few minor typos corrected in the text, and better-looking hyperlinks

International audience; Effective radium-226 concentration ( EC Ra) has been measured in soil samples from seven horizontal and vertical profiles of terrace scarps in the northern part of Kathmandu Valley, Nepal. The samples belong to the Thimi, Gokarna, and Tokha Formations, dated from 50 to 14 ky BP, and represent a diverse fluvio-deltaic sedimentary facies mainly consisting of gravelly to coarse sands, black, orange and brown clays. EC Ra was measured in the laboratory by radon-222 emanation. The samples ( n = 177) are placed in air-tight glass containers, from which, after an accumulation time varying from 3 to 18 days, the concentration of radon-222, radioactive decay product of radium-226 and radioactive gas with a half-life of 3.8 days, is measured using scintillation flasks. The EC Ra values from the seven different profiles of the terrace deposits vary from 0.4 to 43 Bq kg -1, with profile averages ranging from 12 ± 1 to 27 ± 2 Bq kg -1. The values have a remarkable consistency along a particular horizon of sediment layers, clearly demonstrating that these values can be used for long distance correlations of the sediment horizons. Widely separated sediment profiles, representing similar stratigraphic positions, exhibit consistent EC Ra values in corresponding stratigraphic sediment layers. EC Ra measurements therefore appear particularly useful for lithologic and stratigraphic discriminations. For comparison, EC Ra values of soils from different localities having various sources of origin were also obtained: 9.2 ± 0.4 Bq kg -1 in soils of Syabru-Bensi (Central Nepal), 23 ± 1 Bq kg -1 in red residual soils of the Bhattar-Trisuli Bazar terrace (North of Kathmandu), 17.1 ± 0.3 Bq kg -1 in red residual soils of terrace of Kalikasthan (North of Trisuli Bazar) and 10 ± 1 Bq kg -1 in red residual soils of a site near Nagarkot (East of Kathmandu). The knowledge of EC Ra values for these various soils is important for modelling radon exhalation at the ground surface, in particular in the vicinity of active faults. Importantly, the study also reveals that, above numerous sediments of Kathmandu Valley, radon concentration in dwellings can potentially exceed the level of 300 Bq m -3 for residential areas; a fact that should be seriously taken into account by the governmental and non-governmental agencies as well as building authorities.

This paper presents an innovative approach to analyze the transitory response of complex and nonlinear systems, which are excited by non-Gaussian and non-stationary random fields, by solving of a statistical inverse problem with experimental measurements. Based on a double expansion, it is particularly adapted to the modeling of stochastic processes that are only characterized by a relatively small set of independent realizations. First, an adaptation of the classical Karhunen-Loeve expansion is presented. Indeed, for the past fifty years, the use of reduced basis has spread to many scientific fields to condense the statistical properties of stochastic processes, and among these bases, the Karhunen-Loeve basis plays a major role as it allows the minimization of the total mean square error. Such a basis corresponds to the Hilbertian basis that is constructed as the eigenfunctions of the covariance operator of the stochastic process of interest. When the available information about this stochastic process is characterized by a limited set of independent realizations, this covariance function is however unknown. Therefore, there is no reason for the set gathering the eigenfunctions associated with any estimator of the covariance to be still optimal. Secondly, the random vector, which gathers the projection coefficients of the stochastic process on this basis, is characterized using a polynomial chaos expansion approach. The dimension of this random vector being very high (around several hundreds), advanced identification techniques are introduced to allow performing relevant convergence analyses and identifications. The non-Gaussian non-stationary stochastic process is identified using the experimental measurements and consequently, constitutes a realistic stochastic modeling. The proposed method is then applied to the risk assessment of a non-linear structure submitted to seismic loadings, for which measured seismic accelerations are available. REFERENCES R. Ghanem, P. D. Spanos, Stochastic Finite Elements: A Spectral Approach, rev. ed., Dover Publications, New York, 2003. O. Le Maitre, O. Knio, Spectral Methods for Uncertainty Quantification, Springer, 2010. G. Perrin, C. Soize, D. Duhamel, C. Funfschilling, Identification of polynomial chaos representations in high dimension from a set of realizations, SIAM J. Sci. Comput., 34(6), A2917–A2945, 2012. G. Perrin, C. Soize, D. Duhamel, C. Funfschilling, A posteriori error and optimal reduced basis for stochastic processes defined by a finite set of realizations, SIAM/ASA J. Uncertainty Quantification, 2, 745-762 (2014). C. Soize, Identification of high-dimension polynomial chaos expansions with random coefficients for non-Gaussian tensor-valued random fields using partial and limited experimental data., Computer Methods in Applied Mechanics and Engineering, 199 (33-36), 2150–2164, 2010.

International audience; A methodology is presented to quantify uncertainties resulting from the analysis of dynamic tests performed on classic split Hopkinson pressure bar system in order to improve material parameter estimation within the framework of Bayesian inference. Since the experimental setup is imperfectly known, the proposed methodology consists in modeling experimental parameters as random variables. Then, cumulative effects of all experimental uncertainties are estimated by a statistical analysis based on one-dimensional wave interpretation. For each test, results consist in stress and strain-rate given as normal random variables. In addition, an experimental campaign is performed on the aluminum alloy AA7075-O, in order to identify material variability and repeatability of tests. Additional tests in the quasi-static regime are performed at two different temperatures to characterize temperature dependence of behavior. Material parameters of a simple Steinberg-Cochran-Guinan model are then estimated by (i) standard Bayesian inference exploiting data in the quasi-static regime, and (ii) a hierarchical Bayesian model exploiting data in the dynamic regime. The fitted model agrees well with the measurements and model uncertainties are easily quantified. Results are presented in the form of posterior probability density functions. The systematic quantification of uncertainties in dynamic tests opens interesting perspectives to analyze the response of structures and materials to impact.

We present observations of DES16C2nm, the first spectroscopically confirmed hydrogen-free superluminous supernova (SLSN-I) at redshift z~2. DES16C2nm was discovered by the Dark Energy Survey (DES) Supernova Program, with follow-up photometric data from the Hubble Space Telescope, Gemini, and the European Southern Observatory Very Large Telescope supplementing the DES data. Spectroscopic observations confirm DES16C2nm to be at z=1.998, and spectroscopically similar to Gaia16apd (a SLSN-I at z=0.102), with a peak absolute magnitude of U=-22.26$\pm$0.06. The high redshift of DES16C2nm provides a unique opportunity to study the ultraviolet (UV) properties of SLSNe-I. Combining DES16C2nm with ten similar events from the literature, we show that there exists a homogeneous class of SLSNe-I in the UV (~2500A), with peak luminosities in the (rest-frame) U band, and increasing absorption to shorter wavelengths. There is no evidence that the mean photometric and spectroscopic properties of SLSNe-I differ between low (z1), but there is clear evidence of diversity in the spectrum at 2 these events appear optically red, peaking in the observer-frame z-band. Such characteristics are critical to identify these objects with future facilities such as the Large Synoptic Survey Telescope, Euclid, and the Wide-Field Infrared Survey Telescope, which should detect such SLSNe-I to z=3.5, 3.7, and 6.6, respectively., Accepted for publication in ApJ

International audience; In 1948, P. Kirkpatrick and A. V. Baez developed an x-ray microscope (energy range about 100 eV-10 keV) composed of two concave spherical mirrors working at grazing incidence. That device, named KB microscope, presents a 3-5 μm resolution within a field having a radius about 100 μm; outside that field, its resolution lowers rapidly when the object point recedes from the center. The adjunction of two similar mirrors can notably increase the useful field (typically, the resolution can be better than 10 μm within a 2-mm-diam field of view), which is necessary for studying laser plasmas. Its main advantage with respect to more simple optics, as the pinhole, is that it can be located far enough from the plasma to avoid any destruction during the shot. We describe such a microscope that we call KBA microscope and present some images of fine metallic grids. Those grids were backlighted by x-raysources, either a cw one or a series of laser plasmas from the Octal-Héliotrope facility. Examining the films in detail shows that the experimental results are very close to the theoretical characteristics; hence the interest of this device for the x-raydiagnostics on the future powerful laser facilities.

The effect of shear strain on the high‐pressure behavior of nitromethane has been studied by Raman spectroscopy in a shear diamond anvil cell (SDAC). Two major effects of shear strain have been observed. The first one is a lowering of the pressures at which the different spectral transformations occur. The second effect is observed at 28 GPa. At this pressure, a sudden decomposition of the sample occurs due to the shear deformation. Observation of the sample after decomposition shows the presence of a black residue, which is mainly composed of carbon soot.

International audience; We investigate electron trapping by high-amplitude whistler-mode waves propagating at small as well as large angles relative to geomagnetic field lines. The inhomogeneity of the background magnetic field can result in an effective acceleration of trapped particles. Here, we derive useful analytical expressions for the probability of electron trapping by both parallel and oblique waves, paving the way for a full analytical description of trapping effects on the particle distribution. Numerical integrations of particle trajectories allow to demonstrate the accuracy of the derived analytical estimates. For realistic wave amplitudes, the levels of probabilities of trapping are generally comparable for oblique and parallel waves, but they turn out to be most efficient over complementary energy ranges. Trapping acceleration of 100 keV electrons. (C) 2015 AIP Publishing LLC.

International audience; We investigate the radiation effects on germanosilicate optical fiber acting as the sensing element of optical frequency domain reflectometry devices. Thanks to a new setup permitting to control temperature during irradiation, we evaluate the changes induced by 10 keV x rays on their Rayleigh response up to 1 MGy in a temperature range from −40°C up to 75°C. Irradiation at fixed temperature points out that its measure is reliable during both irradiation and the recovery process. Mixed temperature and radiation measurements show that changing irradiation temperature leads to an error in distributed measurements that depends on the calibration procedure. These results demonstrate that Rayleigh-based optical fiber sensors are very promising for integration in harsh environments.

In the present paper, we investigate the trapping of relativistic electrons by intense whistler-mode waves or electromagnetic ion cyclotron waves in the Earth’s radiation belts. We consider the non-resonant impact of additional, lower amplitude magnetic field fluctuations on the stability of electron trapping. We show that such additional non-resonant fluctuations can break the adiabatic invariant corresponding to trapped electron oscillations in the effective wave potential. This destruction results in a diffusive escape of electrons from the trapped regime of motion and thus can lead to a significant reduction of the efficiency of electron acceleration. We demonstrate that when energetic electrons are trapped by intense parallel or very oblique whistler-mode waves, non-resonant magnetic field fluctuations in the whistler-mode frequency range with moderate amplitudes around 3 15 pT (much less intense than the primary waves) can totally disrupt the trapped motion. However, the trapping of relativistic electrons by electromagnetic ion cyclotron waves is noticeably more stable. We also discuss how the proposed approach can be used to estimate the effects of wave amplitude modulations on the motion of trapped particles.

International audience; We describe a mechanism of resonant electron acceleration by oblique high-amplitude whistlerwaves under conditions typical for the Earth radiation belts. We use statistics of spacecraftobservations of whistlers in the Earth radiation belts to obtain the dependence of the angle hbetween the wave-normal and the background magnetic field on magnetic latitude k. According tothis statistics, the angle h already approaches the resonance cone at k 15 and remains close to itup to k 30–40 on the dayside. The parallel component of the electrostatic field of whistlerwaves often increases around k 15 up to one hundred of mV/m. We show that due to thisincrease of the electric field, the whistler waves can trap electrons into the potential well via waveparticle resonant interaction corresponding to Landau resonance. Trapped electrons then move withthe wave to higher latitudes where they escape from the resonance. Strong acceleration is favoredby adiabatic invariance along the increasing magnetic field, which continuously transfers theparallel energy gained to perpendicular energy, allowing resonance to be reached and maintained.The concomitant increase of the wave phase velocity allows for even stronger relative accelerationat low energy

International audience; The packaging is an important step, allowing to transform the transducer chip into a sensor. This step is necessary to perform the sensor characterization in a real environment, while minimizing the impact of the influence parameters on the sensor response. This study concerns the impact of the packaging on the static response of a miniature pressure sensor dedicated to the monitoring of blast waves. The transducer is based on 5 µm-thick rectangular (55 µm x 135 µm) silicon membrane and piezoresistive gauges. Pressure sensitivity measurements are performed using stressed and non-stressed sensor's holder configurations. Measurements results indicate that the pressure sensitivity is doubled when the sensor's holder is stressed. Finite Element Method simulations using COMSOL Multiphysics software show here that this result originates from the deformation of the sensor's holder, which leads to the deflection of the thin silicon membrane.

International audience; Solar flares generate intense electromagnetic radiation in the radio domain (1-100 MHz), part of which corresponds to emission at the electron plasma frequency ωp or its second harmonic 2ω_p. These waves are the signature of electron beams propagating in the interplanetary medium. Detected by space and ground-based radio telescopes, these electromagnetic waves can be used to remotely infer the characteristics of the electron beams. If the main steps of the process from electron beam to Electron Plasma Waves and mode conversion to electromagnetic radiations are known, several questions are still unresolved, in particular: (i) the efficiency of the conversion, (ii) the directionality of the process. To tackle these questions first PIC simulations, then laboratory laser-plasma experiment have been performed. If the parameters of the interplanetary plasma and laser-generated plasmas are very different in absolute values, they happen to be very similar in relative numbers (such as k⪉mbdaDe and T_e/T_i), which makes these laser experiments suitable for astrophysical purposes. The experiment was designed for the LULI 2000 facility (Ecole Polytechnique, France) operated at 2ω0 in the nanosecond regime. The results of PIC simulations and of the first experiment that hold in September 2018 will be presented.

High-intensity laser accelerated protons and ions are emerging sources with complementary characteristics to those of conventional sources, namely high charge, high current, and short bunch duration, and therefore can be useful for dedicated applications. However, these beams exhibit a broadband energy spectrum when, for some experiments, monoenergetic beams are required. We present here an adaptation of conventional chicane devices in a compact form (10 cm × 20 cm) which enables selection of a specific energy interval from the broadband spectrum. This is achieved by employing magnetic fields to bend the trajectory of the laser produced proton beam through two slits in order to select the minimum and maximum beam energy. The device enables a production of a high current, short duration source with a reproducible output spectrum from short pulse laser produced charged particle beams.