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Memory-and compute-optimized geometric multigrid GMGPolar for curvilinear coordinate representations -Applications to fusion plasma
No full textTokamak fusion reactors are actively studied as a means of realizing energy production from plasma fusion. However, due to the substantial cost and time required to construct fusion reactors and run physical experiments, numerical experiments are indispensable for understanding plasma physics inside tokamaks, supporting the design and engineering phase, and optimizing future reactor designs. Geometric multigrid methods are optimal solvers for many problems that arise from the discretization of partial differential equations. It has been shown that the multigrid solver GMGPolar solves the 2D gyrokinetic Poisson equation in linear complexity and with only small memory requirements compared to other state-of-the-art solvers. In this paper, we present a completely refactored and object-oriented version of GMGPolar which offers two different matrix-free implementations. Among other things, we leverage the Sherman-Morrison formula to solve cyclic tridiagonal systems from circular line solvers without additional fill-in and we apply reordering to optimize cache access of circular and radial smoothing operations. With the Give approach, memory requirementsare further reduced and speedups of four to seven are obtained for usual testcases. For the Take approach, speedups of 16 to 18 can be attained
Measuring Modern Phishing Tactics: A Quantitative Study of Body Obfuscation Prevalence, Co-occurrence, and Filter Impact
No full textPhishing attacks frequently use email body obfuscation to bypass detection filters, but quantitative insights into how techniques are combined and their impact on filter scores remain limited. This paper addresses this gap by empirically investigating the prevalence, co-occurrence patterns, and spam score associations of body obfuscation techniques. Analysing 386 verified phishing emails, we quantified ten techniques, identified significant pairwise co-occurrences revealing strategic layering like the presence of text in images with multipart abuse, and assessed associations with antispam scores using multilinear regression. Text in Image (47.0%), Base64 Encoding (31.2%), and Invalid HTML (28.8%) were highly prevalent. Regression (R²=0.486, p<0.001) linked Base64 Encoding and Text in Image with significant antispam evasion (p<0.05) in this configuration, suggesting potential bypass capabilities, while Invalid HTML correlated with higher scores. These findings establish a quantitative baseline for complex evasion strategies, underscoring the need for multi-modal defences against combined obfuscation tactics
Force Feedback in Model-Predictive Control: A Soft Contact Approach
No full textInternational audienceModel-predictive control is an appealing framework to control robots due to its ability to exploit both sensory information and model predictions. But its performance remains fundamentally limited in tasks involving contact with the environment, in part because optimal control policies do not reason over force measurements. In this article, we propose a first complete answer to this issue by introducing a novel approach to perform force feedback in model-predictive control. We propose to augment the state-space with a visco-elastic model of the contact force in the task space in order to systematically include measured efforts into the optimal control loop. We derive a complete predictive controller with an efficient formulation whose implementation is released in open-source. We demonstrate through simulation studies and hardware experiments that our approach enables to combine the benefits of force control and model-predictive control within a single architecture, thereby outperforming existing approaches in challenging contact tasks
Structural comparison of homomolecular systems on surfaces using a fingerprint-based method
No full textInternational audienceThis work presents an adaptation of the Smooth Overlap of Atomic Positions (SOAP) method to improve the efficiency of (dis)similarity quantification in homogeneous molecular (homomolecular) systems. SOAP, a fingerprint-based approach, is widely used to measure molecular similarity. We propose variants of SOAP kernels that leverage the structural architecture of homomolecular systems to minimize irrelevant comparisons of atomic environments. To evaluate its performance, we apply this adapted SOAP-based method to a synthetic dataset consisting of two identical tripeptides deposited on a copper surface, simulating different molecular states. The results demonstrate that the adapted method not only improves computational efficiency but also yields more meaningful clustering outcomes by better capturing the key structural differences between states. These findings suggest that the proposed method is well-suited for the study of homomolecular systems, particularly those involving surface interactions, and has the potential to enhance the use of diverse types of molecular modeling and analysis methods that rely on (dis)similarity measures
This work was supported by Prociencia-Concytec, Perú, under project "Manufactura de Estaciones de Recarga DC Bidireccionales de Vehículos Eléctricos, de alta durabilidad y eficiencia para aplicaciones "Vehicle to Grid" and "Vehicle to Home" en condiciones de funcionamiento de carga equilibradas y/o desequilibradas" under Grant PE501082509-2023. An earlier version of this paper was presented in part at 2023 IEEE Energy Conversion Congress and Exposition (ECCE) [
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Arrival Control in Quasi-Reversible Queueing Systems: Optimization and Reinforcement Learning
No full textIn this paper, we introduce a versatile scheme for optimizing the arrival rates of quasi-reversible queueing systems. We first propose an alternative definition of quasi-reversibility that encompasses reversibility and highlights the importance of the definition of customer classes. In a second time, we introduce balanced arrival control policies, which generalize the notion of balanced arrival rates introduced in the context of Whittle networks, to the much broader class of quasi-reversible queueing systems. We prove that supplementing a quasi-reversible queueing system with a balanced arrival-control policy preserves the quasi-reversibility, and we specify the form of the stationary measures. We revisit two canonical examples of quasi-reversible queueing systems, Whittle networks and order-independent queues. Lastly, we focus on the problem of admission control and leverage our results in the frameworks of optimization and reinforcement learning
Solving unbounded optimal control problems with the moment-SOS hierarchy *
No full textInternational audienceThe behaviour of the moment-sums-of-squares (moment-SOS) hierarchy for polynomial optimal control problems on compact sets has been explored to a large extent. Our contribution focuses on the case of non-compact control sets. We describe a new approach to optimal control problems with unbounded controls, using compactification by partial homogenization, leading to an equivalent infinite dimensional linear program with compactly supported measures. Our results are closely related to the results of a previous approach using DiPerna-Majda measures. However, our work provides a sound proof of the absence of relaxation gap, which was conjectured in the previous work, and thereby enables the design of a moment-sum-of-squares relaxation with guaranteed convergence
Fabrication et caractérisation de diodes Schottky 1 kV avec protection par plaque de champ en Al2O3 /Si3N4
No full textInternational audienceDans un contexte d’électrification des systèmes, l’optimisation des composants électroniques est primordiale pour réduire la consommation énergétique. Parmi les matériaux à large bande interdite étudiés pour remplacer le silicium, le diamant se distingue comme un candidat prometteur pour des applications de haute puissance.Actuellement, la diode Schottky constitue le dispositif en diamant le plus étudié et démontre des résultats encourageants. Toutefois, des améliorations demeurent nécessaires pour accroître la tension de claquage. L’absence de terminaison de jonction efficace provoque notamment une concentration du champ électrique en périphérie du composant, entraînant un claquage électrique prématuré et limitant ainsi les performances théoriques du diamant.La solution de protection périphérique par plaque de champ (Field Plate, FP) est couramment utilisée pour protéger le contact Schottky et réduire le champ électrique au bord du composant. Cependant, de nombreux designs de FP présentent un claquage prématuré du diélectrique en raison de l’utilisation de matériaux à faible permittivité. Pour y remédier, nous proposons un design de FP basé sur un empilement Al₂O₃/Si₃N₄ (20/200 nm), présentant des permittivités élevées. La qualité de l’empilement a été préalablement étudiée par des mesures C-V sur silicium.Le processus de fabrication des diodes sera présenté. La tension de claquage sera évaluée par des mesures I-V réalisées dans un environnement sous vide entre 300 K et 850 K. De plus, la nature et la densité des pièges dans les diélectriques, susceptibles de dégrader les performances des diodes, seront analysées par des mesures C-V sur des capacités MIS (Métal-Isolant-Semi-conducteur), intégrées au même échantillon diamant
Assessment of directional scattering from Mie-resonant-plasmonic Si@Au core-shell nanoparticles
No full textInternational audienceMetamaterial research has sought to create nanostructures with strong directional optical scattering to control light propagation at the nanoscale. Core-shell architectures comprised of both resonant cores and resonant shells have been suggested as candidate particles in which the spectral overlap of the electric and magnetic dipoles can be controlled to create strong directional scattering. In this study, we present Au-decorated Si core-shell (Si@Au) particles. These were synthesized by creating Si particles through the thermal disproportionation of hydrogen silsesquioxane (HSQ), which were then decorated with ∼ 4 nm diameter Au nanoparticles. We characterized the resonant behavior of the core-shell particles using electron energy-loss spectroscopy mapping and optical single-particle scatter spectroscopy. These observations were supported by T-matrix simulations and Mie theory calculations of the scattering spectra, which show that compared to Si, Si@Au particles demonstrate a dampened magnetic dipole resonance for smaller Si core diameters (100 – 130 nm) and an enhanced magnetic dipole resonance for larger Si core sizes (150 – 200 nm). However, we show that continuous plasmonic shells of ~12 nm thickness are needed to significantly improve forward scattering intensity