Modèle par éléments discrets pour l’étude du comportement dynamique d’un matériau élastique.

Le comportement mécanique des matériaux est généralement simulé par des approches issues de la mécanique des milieux continus. Cependant, lorsqu’il s’agit de simuler des phénomènes de multi fissurations voir de multi fracturations, les modèles de la mécanique discrète s’avèrent mieux adaptés, car ils prennent en compte naturellement les discontinuités générées par les interfaces. La difficulté est alors de s’assurer qu’une approche par éléments discrets (DEM) permet bien de retrouver le comportement mécanique au sens de la mécanique des milieux continus. Cet article propose une méthodologie permettant, à partir des données connues du matériau à simuler (module de Young, coefficient de Poisson, célérité de propagation des ondes), de quantifier les paramètres « microscopiques » du modèle DEM

Design of a full silica pulse compression grating

International audienceA diffraction grating engraved on a two-dimensional photonic crystal composed of square air holes in a silica matrix is numerically studied for the compression of ultra-short pulses. The silica is therefore the only solid material of the grating and the reflection of the incident beam is based on the contrast of the air and silica refractive indices. This optical component enables the single use of silica as a solid material presenting a high laser induced damage threshold. In comparison to gratings engraved on a dielectric stack (MLD), it offers the advantage of avoiding the presence of interfaces between 2 solid materials with different mechanical properties, sources of mechanical constraints which can distort the grating

Optical performance and laser induced damage threshold improvement of diffraction gratings used as compressors in ultra high intensity lasers

International audienceThis paper studies gratings engraved in a multilayer dielectric stack for ultra high intensity laser compressors. A metal layer is inserted between the substrate and the dielectric stack to reduce the number of dielectric bilayers and thus the mechanical stress within the stack. A code taking account the fluctuation range of the geometrical parameters during the fabrication process is used to numerically optimize the mirror stack and study different groove profiles to increase the reflected efficiency and the laser induced damage threshold. It is evidenced that of all the profiles leading to good diffraction performances, those with the greatest groove depth and width values result in the smallest enhancement of the electric field square inside the grating with a decrease by a factor close to 2.5

Microscopic evidence of the role of the near field enhancement in the short pulse damage mechanism of diffraction gratings

International audienceWe investigate the short pulse laser induced damage initiation mechanism on multilayer dielectric (MLD) pulse compression gratings. We report by means of scanning electron microscopy that damages initiate on the edge of the grating pillars opposite to the incoming wave. It demonstrates, at the scale of a grating line, the role of the electric field in the damage process but we also that grating pillars damage is also spatially modulated in the form of a periodic ripple pattern developing along the polarization direction

A discrete element model to investigate sub-surface damage due to surface polishing

International audienceLarge high-power laser facilities such as megajoule laser (LMJ) or National Ignition Facility (NIF) are designed to focus about 2 MJ of energy at the wavelength of 351 nm, in the center of an experiment chamber. The final optic assembly of these systems, operating at 351 nm is made of large fused silica optics working in transmission. When submitted to laser at the wavelength of 351 nm, fused silica optics can exhibit damage, induced by the high amount of energy traversing the part. The created damage is a set of micro-chips that appear on the optic surface. Current researches have shown that this damage could be initiated on pre-existing sub-surface damages created during the optics manufacturing process. It is then very important to understand, for various set of manufacturing parameters, what are the key parameters for sub-surface damage. The presented work details the development of a simplified model to investigate the polishing process. Both silica (the material to be polished) and the abrasive particles are modeled using a discrete element approach. This numerical tool allows following the evolution of micro-cracks inside the material during the abrasion process. It is shown how the mechanical properties (pressure), the abrasive properties (shape and quantity of abrasive particles) and the system properties (filtration) have an influence on the sub-surface properties at the end of the process

Delay interferometric single-shot measurement of a petawatt-class laser longitudinal chromatism corrector

International audienceIn this paper we present a self-referenced interferometric single-shot measurement technique that we use to evaluate the longitudinal chromatism compensation made by a diffractive lens corrector. A diffractive lens with a delay of 1 ps is qualified for a 60 mm beam aperture. This corrector was implemented on the Alisé Nd:glass power chain. We qualify the corrector and the Alisé power chain chromatism, demonstrating the potential of this measuring principle as well as the interest of diffractive lenses to correct longitudinal chromatism of petawatt-class lasers

Microplastics contamination in blue mussels (Mytilus edulis (L.)) and marine sediments along the coast of British Columbia, Canada

Contamination of marine ecosystems with microplastics (plastic particles ≤ 5mm) is now recognized as a serious and growing threat to sealife. One major concern is that invertebrates, fish, seabirds and marine mammals mistake plastic for food, leading to suffocation, blockage of the gut and/or malnutrition. Microplastics have been detected in sediment around the world, highlighting the propensity of this matrix to serve as a sink. Due to their extensive filter-feeding activity, marine bivalves are directly exposed to this structural pollutant. In the present study, we investigate microplastics contamination in nearshore subtidal sediment and mussel samples collected at 43 sites along the British Columbia coast. Microplastics were extracted from a 50g sediment subsample using a newly-developed method based on oleophilic properties of microplastics while mussel soft tissues were digested using enzymes. All samples are currently being analyzed under light microscopy to count and characterize (shape, colour, size) microplastics particles. A subset of microplastics will then be analyzed using Fourier-Transform Infrared Spectrometry (FTIR) in order to characterize polymer types. Our results will help shed some light on the source, transport and fate of microplastics in coastal British Columbia

Imaging subsurface damage of grinded fused silica optics by confocal fluorescence microscopy

International audienceWe report an experimental investigation of fluorescence confocal microscopy as a tool to measure subsurface damage on grinded fused silica optics. Confocal fluorescence microscopy was performed with an excitation at the wavelength of 405 nm on fixed abrasive diamond grinded fused silica samples. We detail the measured fluorescence spectrums and compare them to those of oil based coolants and grinding slurries. We evidence that oil based coolant used in diamond grinding induces a fluorescence that marks the subsurface damages and eases its observation. Such residual traces might also be involved in the laser damage process