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Efficiency and beam quality for positron acceleration in loaded plasma wakefields
No full textInternational audienceAccelerating particles to high energies in plasma wakefields is considered to be a promising technique with good energy efficiency and high gradient. While important progress has been made in plasma-based electron acceleration, positron acceleration in plasma has been scarcely studied and a fully self-consistent and optimal scenario has not yet been identified. For high energy physics applications where an electron-positron collider would be desired, the ability to accelerate positrons in plasma wakefields is, however, paramount. Here we show that the preservation of beam quality can be compromised in a plasma wakefield loaded with a positron beam, and a tradeoff between energy efficiency and beam quality needs to be found. For electron beams driving linear plasma wakefields, we have found that despite the transversely nonlinear focusing force induced by positron beam loading, the bunch quickly evolves toward an equilibrium distribution with limited emittance growth. Particle-in-cell simulations show that for μm-scale normalized emittance, the growth of uncorrelated energy spread sets an important limit. Our results demonstrate that the linear or moderately nonlinear regimes with Gaussian drivers provide a good tradeoff, achieving simultaneously energy-transfer efficiencies exceeding 30% and uncorrelated energy spread below 1%, while donut-shaped drivers in the nonlinear regime are more appropriate to accelerate high-charge bunches at higher gradients, at the cost of a degraded tradeoff between efficiency and beam quality
Ultra‐wideband antenna system for in‐band full‐duplex applications
No full textInternational audienceIn this paper, we present an ultra-wideband full-duplex antenna system constituted of a wideband 4-element Vivaldi array and wideband microstrip-to-slotline baluns. The proposed system is characterized by its simplicity, high directivity, and high self-interference cancellation levels over a wide frequency bandwidth. The system is abricated using basic PCB technology and can provide at least 50 dB of self-interference cancellation over the bandwidth of operation, 4 - 40 GHz, with an average gain of 7.8 dBi. The system has a size of 8 cm x 8 cm x 9.4 cm and can be used as a high data rate link between two distant wireless nodes. To assess the merits of the proposed system and compare it to other published works, a new figure of merit (FoMWFD) dedicated to wide-band full-duplex antenna topologies is introduced in this paper
Low-Complexity Blind PAPR Reduction for OFDM Systems with Rotated Constellations
No full textInternational audienceThe high Peak-to-Average Power Ratio (PAPR) of Orthogonal Frequency Division Multiplexing (OFDM) signals leads to a serious system performance degradation. To work around this issue, several algorithms have been proposed in the literature to reduce the PAPR, but, they often suffer from multiple limitations; in particular, the main issue with interleaving techniques is the spectral efficiency loss, as the transmission of a Side Information (SI) is generally required. In contrast to previous works, this paper proposes a blind interleaving technique for OFDM systems with signal space diversity. Indeed, with Rotated and Cyclically Q-Delayed (RCQD) constellations, the In-phase (I) and Quadrature (Q) components of constellations symbols are correlated, which allows the receiver to estimate the interleaver index without any SI. Moreover, to lower down the complexity burden at the receiver side, we first design a blind decoder based on the Minimum Mean Square Error (MMSE) criterion and we then propose a low complexity decoder for the Uniformly Projected RCQD (UP-RCQD) QAM, as this constellation has several interesting structural properties and achieves near optimum BER performance. Simulation results show that our proposal leads to a large PAPR reduction and to a near optimum BER performance that outperforms, over various channels, the solution currently used in DVB-T2. They also underline the good performance of the blind decoding performed with up to 98% of complexity reduction compared to the max-log Maximum Likelihood (ML) estimation
Digital micromirror device for holographic and Fourier optics applications
No full textInternational audienceThe electromagnetic wavefront diffracted by an object carries information about the shape of the object from which the wave was emitted. Being able to record the phase and intensity of such a wave thus allows to reconstruct the object from the information carried by the wave, even if the object is no longer present. Among the reconstruction techniques, holography plays a big part. However the waves may experience a great variety of distortions on their way from the object to the measurement apparatus. Thus being able to shape the wavefront at will is key in holography. Micromirror light modulators are powerful tools for that matter and are well known for holographic applications. This paper explores the fundamental principles for digitally reconstructing a precise image of an object, but also for digitally correcting an imperfectly shaped wavefront, by exploiting the diffraction properties of light on a reflective surface. The methods presented here have been implemented as part of practical work for 2nd year students at the Ecole Polytechnique (last year of undergraduate program)
MAESSTRO 1.0.0
No full textMAESSTRO is a software for sound synthesis of piano tones based on a complete vibroacoustic modeling of the piano. The main goal is to provide piano makers a set of tools that allow them to predict the acoustic impacts of structural modifications of piano soundboards on the resulting sounds. The interest of the physical modeling that drives the numerical computations embedded in the software is to be able to reproduce the vibroacoustics of the piano in a realistic fashion: the acoustic variations between tones produced by different pianos faithfully reflect their structural differences. For that purpose, MAESSTRO gathers and coordinates several numerical and physical models that numerically simulate all of the physical phenomena involved in the production of piano tones, from the hammer impulsion upto the acoustic radiation.MAESSTRO includes several functionalities which are the following* entering the geometry and the materials of the virtual soundboard thanks to a Graphical User Interface (GUI), * feeding MAESSTRO with MIDI files to be synthesized,* simulating numerically the physical phenomena involved in the production of piano tones, * post-processing the software outputs, * creating audio files of synthesized piano tones.The package contains three subpackages :* maesstro-ihm contains the source codes of the GUI, implemented in TypeScript* maesstro-matlab is a Matlab toolbox that contains the source codes for the computation of the modal basis of the soundboard, including an automatic mesh generator using GMSH. Matlab is only needed to modify the source files and generate updated executables. In order to launch the executables, only Matlab Runtime is needed* maesstro-python contains the python modules which is used to call the different modules via a command line interfac
Analytical preconditioners for Neumann elastodynamic Boundary Element Methods
No full textInternational audienceRecent works in the Boundary Element Method (BEM) community have been devoted to the derivation of fast techniques to perform the matrix vector product needed in the iterative solver. Fast BEMs are now very mature. However, it has been shown that the number of iterations can significantly hinder the overall efficiency of fast BEMs. The derivation of robust preconditioners is now inevitable to increase the size of the problems that can be considered. Analytical precon-ditioners offer a very interesting strategy by improving the spectral properties of the boundary integral equations ahead from the discretization. The main contribution of this paper is to propose new analytical preconditioners to treat Neumann exterior scattering problems in 2D and 3D elasticity. These preconditioners are local approximations of the adjoint Neumann-to-Dirichlet map. We propose three approximations with different orders. The resulting boundary integral equations are preconditioned Combined Field Integral Equations (CFIEs). An analytical spectral study confirms the expected behavior of the preconditioners, i.e., a better eigenvalue clustering especially in the elliptic part contrary to the standard CFIE of the first-kind. We provide various 2D numerical illustrations of the efficiency of the method for different smooth and non smooth geometries. In particular, the number of iterations is shown to be independent of the density of discretization points per wavelength which is not the case of the standard CFIE. In addition, it is less sensitive to the frequency
Les dynamiques en cours dans les écoles d'ingénieurs au Maghreb face aux défis environnementaux
No full textInternational audienceBasée sur une enquête qualitative par entretiens réalisée entre 2018 et 2021, cette communication présente les grands enjeux concernant la formation des ingénieurs tels qu’ils sont discutés dans es sphères décisionnelles du Maroc, de l'Algérie et de la Tunisie. Son apport se situe dans l’approche qualitative des grands enjeux qui touchent les politiques de formations d’ingénieurs dans les trois pays du Maghreb, basée non pas sur une comparaison d’indicateurs mais sur les visions des responsables de l’enseignement supérieur de ces pays, recueillies par entretiens. Son originalité tient également à la démarche, inscrite dans la sociologie du curriculum, qui vise à comprendre le processus de construction des curricula à travers les enjeux et les transactions sociales à l’œuvre dans le champ éducatif. Cette approche et cette démarche permettent, à travers cette communication, de mettre en lumière les visions politiques des défis qui concernent les formations d’ingénieurs au Maghreb et la manière dont elles contribuent à la production curriculaire. L’approche comparative entre les trois pays permet de saisir les dynamiques en cours à l’échelle du Maghreb, première étape avant l’identification de leviers d’action pour préparer au mieux les ingénieurs de demain à répondre aux défis environnementaux
