HAL-Polytechnique
Not a member yet
51406 research outputs found
Sort by
Towards calibration of picosecond O-TALIF
No full textInternational audienceExperimental procedure of obtaining the Xe/O two-photon absorption cross-section ratio is discussed for nanosecond and picosecond TALIF experiments. The same nanosecond capillary discharge with 100% oxygen dissociation at 30 mbar pressure is used as a source of O-atoms
Intuitionistic BV
No full textInternational audienceWe present the logic IBV, which is an intuitionistic version of BV, in the sense that its restriction to the MLL connectives is exactly IMLL, the intuitionistic version of MLL. For this logic we give a deep inference proof system and show cut elimination. We also show that the logic obtained from IBV by dropping the associativity of the new non-commutative seq-connective is an intuitionistic variant of the recently introduced logic NML. For this logic, called INML, we give a cut-free sequent calculus
Strong-field ionization in particle-in-cell simulations
No full textInternational audienceThe inclusion of the process of multiple ionization of atoms in high-intensity electromagnetic fields into particle-in-cell (PIC) codes applied to the simulation of laser-plasma interactions is a challenging task. In this paper, we first revisit ionization rates as given by the Smirnov-Chibisov and Perelomov-Popov-Terent'yev formulas within the paradigm of sequential tunnel ionization. We analyze the limit of validity and possible inconsistencies of this approach. We show that a strongly limiting factor to a precise description of ionization is the competing contribution of different sequential ionization processes. To solve this an algorithm is proposed that allows one to find the dominant nonsequential path of tunnel ionization and significantly improves the precision in simulations. This procedure is implemented in the PIC code SMILEI, and includes the dependence of the ionization rates on the magnetic quantum number of the level. The sensitivity to variations in the ionization model is studied via full simulations of the ionization of an argon target by an incident high-intensity laser pulse. Finally, we analyze generalizations of the Perelomov-Popov-Terent'yev rate developed to describe the barrier suppression ionization in high fields and discuss the necessity and possibility of including these extensions in PIC simulations
Laser driven FLASH radiobiology using a high dose and ultra high dose rate single pulse proton source
No full textInternational audienceLaser-driven proton sources have long been developed with an eye on their potential for medical application to radiation therapy. These sources are compact, versatile, and show peculiar characteristics such as extreme instantaneous dose rates, short duration and broad energy spectrum. Typical temporal modality of laser-driven irradiation, the so-called fast-fractionation, results from the composition of multiple, temporally separated, ultra-short dose fractions. In this paper we present the use of a high-energy laser system for delivering the target dose in a single nanosecond pulse, for ultra-fast irradiation of biological samples. A transport line composed by two permanent-magnet quadrupoles and a scattering system is used to improve the dose profile and to control the delivered dose-per-pulse. A single-shot dosimetry protocol for the broad-spectrum proton source using Monte Carlo simulations was developed. Doses as high as 20 Gy could be delivered in a single shot, lasting less than 10 ns over a 1 cm diameter biological sample, at a dose-rate exceeding 10 9 Gy s -1 . Exploratory application of extreme laser-driven irradiation conditions, falling within the FLASH irradiation protocol, are presented for irradiation in vitro and in vivo. A reduction of radiation-induced oxidative stress in vitro and radiation-induced developmental damage compatible with the onset of FLASH effect were observed in vivo, whereas anti-tumoral efficacy was confirmed by cell survival assay
Impact of (NH4)2SO4 agricultural atmospheric pollutant on the degradation mechanisms of thin layer Cu(In,Ga)Se2 solar cells
No full textInternational audienceChemical environment is usually disregarded in stability evaluations of photovoltaic devices but it can be critical for agrivoltaics (dual land use for energy production and agriculture). This work considers for the first time the effect of (NH4)2SO4 agricultural pollutant on the chemical degradation mechanisms of thin layer solar cells. Cu(In,Ga)Se2 (CIGS) cells with the architecture SLG/Mo/CIGS/Zn(O,S)/ZnMgO/Al:ZnO/NiAlNi and representative stacks were characterized at different times of temperature/humidity cyclic aging with and without (NH4)2SO4. The pollutant strongly increased degradation rate and modified the key degradation mechanisms. Cu(In,Ga)Se2-absorber and its interface with Mo-back contact were the most affected by accelerated aging without (NH4)2SO4, while corrosion of front NiAlNi contacts and pitting of window Al-doped ZnO layer was the key degradation mechanism in the presence of (NH4)2SO4. Chemical modifications of the cell and layers were coherent with optoelectrical characteristics loss. The work implies the necessity to take into account specific agricultural pollutants in reliability evaluation of new technologies for agrivoltaic applications
Differential pumping for kHz operation of a laser wakefield accelerator based on a continuously flowing hydrogen gas jet
No full textInternational audienceLaser-Wakefield Accelerators (LWFAs) running at kHz repetition rates hold great potential for applications. They typically operate with lowenergy, highly compressed laser pulses focused on high-pressure gas targets. Experiments have shown that the best-quality electron beams are achieved using hydrogen gas targets. However, continuous operation with hydrogen requires a dedicated pumping system. In this work, we present a method for designing a differential pumping system, which we successfully implemented in our experiments. This enabled the first demonstration of continuous operation of a kHz LWFA using a high-pressure hydrogen gas jet. The system effectively maintained a pressure below 3 × 10 -4 mbar, even with a free-flowing gas jet operating at 140 bar backing pressure. Numerical fluid dynamics and optical simulations were used to guide and validate the system's design.</div
Fatigue Performance of Cemented Aeolian Sands: Linking Material Structural Characteristics to Long-Term Subgrade Durability
No full textInternational audienceIn arid and semi-arid regions, subgrades often consist of loose aeolian sands with insufficient bearing capacity, making effective stabilization essential. Among various techniques, soil–cement columns present a viable solution; however, their long-term performance under cyclic loading is frequently neglected in design considerations. This study investigated the fatigue behavior of cement-treated aeolian sands, representative of soil–cement column materials, subjected to uniaxial cyclic compressive loading, focusing on the effects of cement content anddry density. Cylindrical specimens were prepared with cement contents of 2%, 3%, and 4% (by dry weight) and two dry densities corresponding to relative densities of 10% and 70% in uncemented sand. Cyclic tests were conducted under sinusoidal loading with stress levels ranging from 70% to 98% of the monotonic strength. Fatigue life as well as axial strains were measured, while Scanning Electron Microscopy (SEM) was employed to examine the microstructural features. The results demonstrated that increasing both cement content and density significantly enhances fatigue life. Microstructural analyses revealed that, beyond the quantity of bonds, the spatial distribution and morphology of interparticle bonds critically influence fatigue resistance. Additionally, the evolution and accumulation of dissipated energy were found to be closely associated with fatigue life and applied stress levels. A predictive model was proposed based on total cumulative dissipated energy and the porosity/cement index, providing a practical tool for assessing the durability of stabilized sands. These findingscontribute to a deeper understanding of fatigue mechanisms in cemented aeolian sands and support the design of more resilient subgrades in dry regions
Adaptive Algorithms for Infinitely Many-Armed Bandits: A Unified Framework
No full textWe consider a bandit problem where the buget is smaller than the number of arms, which may be infinite. In this regime, the usual objective in the literature is to minimize simple regret. To analyze broad classes of distributions with potentially unbounded support, where simple regret may not be well-defined, we take a slightly different approach and seek to maximize the expected simple reward of the recommended arm, providing anytime guarantees.To that end, we introduce a distribution-free algorithm, \OSE, that adapts to the distribution of arm means and achieves near-optimal rates for several distribution classes. We characterize the sample complexity through the rank-corrected inverse squared gap function. In particular, we recover known upper bounds and transition regimes for less or greater than when the quantile function is . We additionally identify new transition regimes depending on the noise level relative to , which we conjecture to be nearly optimal.Additionally, we introduce an enhanced practical version, \PROSE, that achieves state-of-the-art empirical performance for the main distribution classes considered in the literature
Generalization in Representation Models via Random Matrix Theory: Application to Recurrent Networks
No full textInternational audienceWe first study the generalization error of models that use a fixed feature representation (frozen intermediate layers) followed by a trainable readout layer. This setting encompasses a range of architectures, from deep random-feature models to echo-state networks (ESNs) with recurrent dynamics. Working in the high-dimensional regime, we apply Random Matrix Theory to derive a closed-form expression for the asymptotic generalization error. We then apply this analysis to recurrent representations and obtain concise formula that characterize their performance. Surprisingly, we show that a linear ESN is equivalent to ridge regression with an exponentially time-weighted ("memory") input covariance, revealing a clear inductive bias toward recent inputs. Experiments match predictions: ESNs win in low-sample, short-memory regimes, while ridge prevails with more data or long-range dependencies. Our methodology provides a general framework for analyzing overparameterized models and offers insights into the behavior of deep learning networks
The Geant4 software toolkit evolution over the past decade
No full textInternational audienceGeant4 is an open software toolkit for the Monte-Carlo simulation of particle transport in matter. It is used in many domains, like high energy and nuclear physics, medical and space science, as well as homeland security and material science. Started 30 years ago, it is still subject to very active developments. This paper presents the developments in Geant4 over the past 10 years, covering the kernel, physics models, and software aspects. It also discusses ongoing R&D projects.</jats:p