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Broadband shock vibration absorber based on vibro-impacts and acoustic black hole effect
No full textInternational audienceA vibration absorber combining the acoustical black hole (ABH) effect with vibro-impacts is presented as a separate device that can be mounted on a primary structure for passive vibration mitigation. The vibration isolator is shown to be effective for low-frequency shock vibration attenuation. Numerical and experimental results are reported, considering a rectangular honeycomb plate as the primary structure, on which the device, consisting of a circular ABH plate with one or several impact masses, is attached. A numerical model is developed using a finite element space discretisation combined with a conservative scheme in a penalty approach for the contact dynamics. The analysis shows that the device has broadband efficiency thanks to the combined effects of energy transfer due to impacts and high attenuation in mid and high-frequency range due to the ABH effect. A detailed parametric study underlines that the tuning of the linear fundamental frequency of the impact mass plays an important role. Besides, considering several impact masses is shown to improve the efficacy. An experimental setup is then used in order to demonstrate the effectiveness of the device. The results confirm the vibration mitigation trends obtained from the numerical model that could then be used as a designed tool for the attenuator. Finally, an analysis based on the Shock Response Spectrum (SRS), widely used in the field of aerospace engineering, is performed, underlining the attenuator is able to reduce rapidly and efficiently the vibrations due to a mechanical shock
Positron Acceleration in Plasma Wakefields
No full textInternational audiencePlasma acceleration has emerged as a promising technology for future particle accelerators, particularly linear colliders. Significant progress has been made in recent decades toward high-efficiency and high-quality acceleration of electrons in plasmas. However, this progress does not generalize to acceleration of positrons, as plasmas are inherently charge asymmetric. Here, we present a comprehensive review of historical and current efforts to accelerate positrons using plasma wakefields. Proposed schemes that aim to increase the energy efficiency and beam quality are summarised and quantitatively compared. A dimensionless metric that scales with the luminosity-per-beam power is introduced, indicating that positron-acceleration schemes are currently below the ultimate requirement for colliders. The primary issue is electron motion; the high mobility of plasma electrons compared to plasma ions, which leads to non-uniform accelerating and focusing fields that degrade the beam quality of the positron bunch, particularly for high efficiency acceleration. Finally, we discuss possible mitigation strategies and directions for future research
Study of a degenerate non-elliptic equation to model plasma heating
No full textInternational audienceIn this manuscript, we study solutions to resonant Maxwell's equations in heterogeneous plasmas. We concentrate on the phenomenon of upper-hybrid heating, which occurs in a localized region where electromagnetic waves transfer energy to the particles. In the 2D case, it can be modelled mathematically by the partial differential equation − div (α∇u) − ω 2 u = 0, where the coefficient α is a smooth, sign-changing, real-valued function. Since the locus of the sign change is located within the plasma, the equation is non-elliptic, and degenerate. On the other hand, using the limiting absorption principle, one can build a family of elliptic equations that approximate the degenerate equation. Then, a natural question is to relate the solution of the degenerate equation, if it exists, to the family of solutions of the elliptic equations. For that, we assume that the family of solutions converges to a limit, which can be split into a regular part and a singular part, and that this limiting absorption solution is governed by the non-elliptic equation introduced above. One of the difficulties lies in the definition of appropriate norms and function spaces in order to be able to study the non-elliptic equation and its solutions. As a starting point, we revisit a prior work [13] on this topic by A. Nicolopoulos, M. Campos Pinto, B. Després and P. Ciarlet Jr., who proposed a variational formulation for the plasma heating problem. We improve the results they obtained, in particular by establishing existence and uniqueness of the solution, by making a different choice of function spaces. Also, we propose a series a numerical tests, comparing the numerical results of Nicolopoulos et al to those obtained with our numerical method, for which we observe better convergence
Total synthesis of photoactivatable latrunculin B for actin-targeting chemical biology
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Spectrum of the Laplacian with mixed boundary conditions in a chamfered quarter of layer
No full textInternational audienceWe investigate the spectrum of a Laplace operator with mixed boundary conditions in an unbounded chamfered quarter of layer. This problem arises in the study of the spectrum of the Dirichlet Laplacian in thick polyhedral domains having some symmetries such as the so-called Fichera layer. The geometry we consider depends on two parameters gathered in some vector κ = (κ_1,κ_2) which characterizes the domain at the edges. We identify the essential spectrum and establish different results concerning the discrete spectrum with respect to κ. By exchanging the axes and/or modifying their orientations if necessary, it is sufficient to restrict the analysis to the cases κ_1\ge0 and κ_2∈ [−κ_1,κ_1]. We identify the essential spectrum and establish different results concerning the discrete spectrum with respect to κ. In particular, we show that for a given κ_1 > 0, there is some h(κ_1) > 0 such that discrete spectrum exists for κ_2 ∈ (−κ_1,0) ∪ (h(κ_1),κ_1) whereas it is empty for κ_2 ∈ [0; h(κ_1)]. The proofs rely on classical arguments of spectral theory such as the max-min principle. The main originality lies rather in the delicate use of the features of the geometry
Impact of gravitational force on high repetition rate filamentation of femtosecond laser pulses in the atmosphere
No full textInternational audienceWe study the influence of the gravitational force on the generation of low-density channels of air left in the path of femtosecond laser filaments at high repetition rate. We observe a more important density variation along the filament longitudinal axis in the case of a vertically created filament as compared to a horizontal one. This leads to a more important reduction of the electrical breakdown field using vertical filament. This geometry induced difference is only observed at high repetition rate because it is directly related to the cumulative effect appearing above 100 Hz
The beneficial role of curiosity on route memory in children
No full textInternational audienceIntroduction : There has been a growing interest in the role of innate curiosity on facets of human cognition, such as in spatial learning and memory. Yet, it is unclear how state level curiosity evoked by the current environment could interact differentially with trait curiosity, to impact spatial memory performance. Methods We assessed the influence of trait and state curiosity on route memory. Forty-two 10-year-old children with low and high-trait curiosity (20 Females; 22 Males) actively explored virtual environments that elicited varying levels of uncertainty (i.e., state-curiosity). Results : As trait curiosity increased, so did memory performance in low and high uncertainty conditions, suggesting that high-curiosity children can better recruit cognitive resources within non-optimal environments. Children with high compared to low trait curiosity also reported greater feelings of presence during exploration. Importantly, in environments with medium uncertainty, children with low trait curiosity were able to perform as well as those with high curiosity. Discussion : Results show that individual differences in trait curiosity influence route learning and these interact dynamically with state-curiosity invoked within different environments
Évaluation de l'intégration des technologies d'intelligence artificielle dans le secteur de la défense et de l'effet de la performance du système national d'innovation sur son développement
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NECO: NEURAL COLLAPSE BASED OUT-OF-DISTRIBUTION DETECTION
No full textInternational audienceDetecting out-of-distribution (OOD) data is a critical challenge in machine learning due to model overconfidence, often without awareness of their epistemological limits. We hypothesize that "neural collapse", a phenomenon affecting in-distribution data for models trained beyond loss convergence, also influences OOD data. To benefit from this interplay, we introduce NECO, a novel post-hoc method for OOD detection, which leverages the geometric properties of "neural collapse" and of principal component spaces to identify OOD data. Our extensive experiments demonstrate that NECO achieves state-of-the-art results on both small and large-scale OOD detection tasks while exhibiting strong generalization capabilities across different network architectures. Furthermore, we provide a theoretical explanation for the effectiveness of our method in OOD detection. Code is available at https://gitlab.com/drti/neco
A Simple Calibration Method to Consider Plastic Deformation Influence on X-ray Elastic Constant Based on Peak Width Variation
No full textInternational audienceThe sin²ψ method is the general method for analyzing X-ray diffraction stress measurements. This method relies on the estimation of a parameter known as 12S2hkl, which is generally considered as a material constant. However, various studies have shown that this parameter can be affected by plastic deformation leading to proportional uncertainties in the estimation of stresses. In this paper, in situ X-ray diffraction measurements are performed during a tensile test with unloads on a low-carbon high-strength steel. The calibrated 12S2hkl parameter varies from 3.5×10−6 MPa−1 to 5.5 ×10−6 Mpa−1, depending on the surface condition and on the plastic strain state, leading to a maximum error on the stress level of 40% compared to reference handbook values. The results also show that plastic strain is responsible for 6 to 14% of the variation, depending on the initial surface sample condition. A method is then proposed to correct this variation based on the fit of the 12S2hkl evolution with respect to the peak diffraction width, the latter being an indication of the plasticity state. It is shown that the proposed methodology improves the applied stress increment prediction, although the absolute stress value still depends on pseudo-macrostresses that also vary with plastic strain