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Modèle numérique du propulseur de satellite PEGASES
No full textThe electric spacecraft propulsion industry is actively transitioning to new propellants.Until recently, the favoured propellant was xenon. It is the heaviest stable noble gas, characteristics that enhance the thrust-to-power ratio of electric thrusters. However, the limited supply cannot satisfy the growing demand as space industrializes.New propulsion systems are designed around lighter noble gases, trading efficiency for affordability.Others make use of molecular propellants, namely iodine. Despite being reactive, this element, a neighbour of xenon in the periodic table, can offer similar performances with the benefit of a higher storage density.The development of the next propulsion systems requires design and simulation tools adapted to alternative propellants. In this work, we propose using a 1D Particle-In-Cell code coupled with a fluid model as a fast way to simulate the low-pressure discharges found in electric thrusters.We implemented an analytical model to emulate the particle transport in the unsimulated directions.This method allows the simulation of simple geometries with a 1D model. In addition, the vacuum permittivity scaling technique allows to speed up whole device simulations.To ensure the accuracy of our model, we extensively validated it using the diagnostic data measured on the PEGASES thruster. This validation process covered a range of noble gases and iodine ICP discharges, including argon, krypton, xenon.Noble gas validation showed that the code could reproduce the trends in the electron parameters as the pressure and power evolved. However, its reduced dimensionality and the fluid model hinder its predictive power at low pressure and high power. In iodine, the low-pressure simulations are in good agreement with the experimental data. However, the model struggles to maintain the delicate balance between the numerous species at high pressure.L'industrie de la propulsion spatial électrique est en train de transitionner vers de nouveaux ergols.Jusqu'à récemment, le xénon était privilégié. Il s'agit du plus lourd des gaz nobles stable, caractéristiques qui bénéficient au rapport poussée/puissance des propulseurs électriques.Cependant, l'offre limitée ne peut satisfaire la demande croissante à mesure que l'espace s'industrialise.De nouveaux systèmes de propulsion sont conçus autour de gaz nobles plus légers, sacrifiant l'efficacité pour le prix. D'autres utilisent des ergols moléculaires, notamment l'iode.Bien que réactif, cet élément voisin du xénon dans le tableau périodique peut offrir des performances similaires avec l'avantage d'une plus grande densité de stockage.Le développement des prochains systèmes de propulsion nécessite des outils de conception et de simulation adaptés aux ergols alternatifs. Dans ce travail, nous proposons d'utiliser un code 1D Particle-In-Cell couplé à un modèle de fluide comme moyen rapide de simuler les décharges à basse pression que l'on trouve dans les moteurs électriques. Nous avons mis en œuvre un modèle analytique pour émuler le transport des particules dans les directions non simulées.Cette méthode permet de simuler des géométries simples avec un modèle 1D. En outre, la technique de changement d'échelle de la permittivité du vide permet de simuler rapidement des systèmes complets.Pour garantir la précision de notre modèle, nous l'avons validé de manière approfondie en utilisant les diagnostic mesurées sur le propulseur PEGASES. Ce processus de validation a couvert des décharges inductives avec de l'iode et divers gaz nobles, l'argon, le krypton et le xénon.La validation des gaz rares a montré que le code pouvait reproduire les tendances des variations des paramètres électroniques au fur et à mesure de l'évolution de la pression et de la puissance.Cependant, sa dimensionnalité réduite et le modèle fluide, entravent son pouvoir prédictif aux plus basses pressions et a trop haute puissance. Pour l'iode, les simulations à basse pression sont en bon accord avec les données expérimentales. Cependant, le modèle peine à maintenir l'équilibre délicat entre les nombreuses espèces à haute pression
Prospects for gravitational wave and ultra-light dark matter detection with binary resonances beyond the secular approximation
No full textInternational audiencePrecision observations of orbital systems have recently emerged as a promising new means of detecting gravitational waves and ultra-light dark matter, offering sensitivity in new regimes with significant discovery potential. These searches rely critically on precise modeling of the dynamical effects of these signals on the observed system; however, previous analyses have mainly only relied on the secularly-averaged part of the response. We introduce here a fundamentally different approach that allows for a fully time-resolved description of the effects of oscillatory metric perturbations on orbital dynamics. We find that gravitational waves and ultra-light dark matter can induce large oscillations in the orbital parameters of realistic binaries, enhancing the sensitivity to such signals by orders of magnitude compared to previous estimates
Investigation of filamentary and diffuse DBD in CO 2 by means of in-situ FTIR absorption spectroscopy
No full textInternational audienceThis work investigates CO2 dielectric barrier discharges at atmospheric pressure in the filamentary and diffuse regimes for the first time using in situ FTIR absorption measurements. The conversion factor of CO2 is determined and is consistent with the results obtained for DBDs in the literature. Vibrational temperatures for CO2 and CO molecules are also determined, as well as the rotational temperature. The ordering of the different temperatures is similar to the reported results for other CO2 discharges. The evolution of the measured parameters as a function of the specific energy input is discussed and a comparison of the two different regimes is carried out
The impact of diffuse Galactic emission on direction-independent gain calibration in high-redshift 21 cm observations
No full textInternational audienceThis study examines the impact of diffuse Galactic emission on sky-based direction-independent (DI) gain calibration using realistic forward simulations of Low-Frequency Array (LOFAR) observations of the high-redshift 21 cm signal of neutral hydrogen during the Epoch of Reionization (EoR). We simulated LOFAR observations between 147 - 159 MHz using a sky model that includes a point source catalogue and diffuse Galactic emission. The simulated observations were DI-gain calibrated with the point source catalogue alone, utilising the LOFAR-EoR data analysis pipeline. A full power spectrum analysis was conducted to assess the systematic bias introduced by DI-gain calibration using complete and incomplete sky models, relative to thermal noise. Additionally, the cross-coherence between observation pairs was computed to determine whether DI-gain calibration errors are coherent or incoherent in specific regions of power spectrum space as a function of integration time. We find that DI-gain calibration with an incomplete sky model that omits diffuse Galactic emission introduces a systematic bias in the power spectrum for bins 0.2 are largely incoherent and average down as noise. We conclude that missing diffuse Galactic emission in the sky model is not a significant contributor to the excess noise observed in the current LOFAR-EoR upper limit results on the 21 cm signal power spectrum
Ultralow-Temperature Thermodynamics and Optical Coherence of Narrow Linewidth Optical Emitters
No full textThe coherence properties of optical emitters in crystals are critical for quantum technologies and optical frequency metrology. Cooling to sub-kelvin temperatures can significantly enhance their coherence, making it essential to identify the key parameters governing emitter and host crystal behavior in this ultra cold regime. We investigate a Czochralski-grown europium doped yttrium orthosilicate crystal, and we report measurements of the heat capacity, a parameter fundamental to evaluating thermal noise limits in metrology schemes based on spectral hole stabilization in such samples. In parallel, we characterize optical coherence via photon echo measurements as a function of temperature. Below 1 K, where phonon contributions diminish, two-level systems (TLS) associated with crystal imperfections may emerge as a limiting factor. A linear-in-temperature term in the heat capacity serves as a signature of TLS, and from our data, we establish an upper bound on this contribution. This, combined with the optical homogeneous linewidth from photon-echo measurements being constant in the interval from 300 mK to 2 K demonstrates a minimal TLSrelated effects in our sample. These findings highlight the promise of ultralow-temperature operation for enhancing the performance of optical quantum devices based on doped crystals
Rapid Construction of Joint Pulsar Timing Array Datasets: The Lite Method
No full textInternational audienceThe International Pulsar Timing Array (IPTA)'s second data release (IPTA DR2) combines observations of 65 millisecond pulsars from 7 radio telescopes spanning decades, aiming to detect nanohertz gravitational waves (GWs). IPTA datasets are complex and take years to assemble, often excluding recent data crucial for low-frequency GW searches. To address this, we introduce the ``Lite'' analysis, a framework that informs the full data combination process. Using a Figure of Merit, we first select individual PTA datasets per pulsar, enabling immediate access to new data and providing an early estimate of fully combined dataset results. Applying this method to IPTA DR2, we create an uncombined dataset (DR2 Lite) and an early-combined subset (EDR2) before constructing the final Full DR2 dataset (IPTA DR2). We find that DR2 Lite can detect the common red noise process seen in Full DR2 but overestimates the amplitude as at , likely due to unmodeled noise. In contrast, the combined datasets improve spectral characterization, with Full DR2 yielding an amplitude of at . Furthermore, combined datasets yield higher, albeit small, detection statistics for Hellings-Downs correlations. Looking ahead, the Lite method will enable rapid synthesis of the latest PTA data, offering preliminary GW constraints before full dataset combinations are available while also motivating their construction
LiteBIRD Science Goals and Forecasts: constraining isotropic cosmic birefringence
No full textInternational audienceCosmic birefringence (CB) is the rotation of the photons' linear polarisation plane during propagation. Such an effect is a tracer of parity-violating extensions of standard electromagnetism and would probe the existence of a new cosmological field acting as dark matter or dark energy. It has become customary to employ cosmic microwave background (CMB) polarised data to probe such a phenomenon. Recent analyses on Planck and WMAP data provide a hint of detection of the isotropic CB angle with an amplitude of around at the level of to . In this work, we explore the LiteBIRD capabilities in constraining such an effect, accounting for the impact of the more relevant systematic effects, namely foreground emission and instrumental polarisation angles. We build five semi-independent pipelines and test these against four different simulation sets with increasing complexity in terms of non-idealities. All the pipelines are shown to be robust and capable of returning the expected values of the CB angle within statistical fluctuations for all the cases considered. We find that the uncertainties in the CB estimates increase with more complex simulations. However, the trend is less pronounced for pipelines that account for the instrumental polarisation angles. For the most complex case analysed, we find that LiteBIRD will be able to detect a CB angle of with a statistical significance ranging from to , depending on the pipeline employed, where the latter uncertainty corresponds to a total error budget of the order of
Cross-national comparison of French and US school food policy and implications
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Euclid Quick Data Release (Q1), A first look at the fraction of bars in massive galaxies at
No full textInternational audienceStellar bars are key structures in disc galaxies, driving angular momentum redistribution and influencing processes such as bulge growth and star formation. Quantifying the bar fraction as a function of redshift and stellar mass is therefore important for constraining the physical processes that drive disc formation and evolution across the history of the Universe. Leveraging the unprecedented resolution and survey area of the Euclid Q1 data release combined with the Zoobot deep-learning model trained on citizen-science labels, we identify 7711 barred galaxies with in a magnitude-selected sample spanning . We measure a mean bar fraction of , consistent with prior studies. At fixed redshift, massive galaxies exhibit higher bar fractions, while lower-mass systems show a steeper decline with redshift, suggesting earlier disc assembly in massive galaxies. Comparisons with cosmological simulations (e.g., TNG50, Auriga) reveal a broadly consistent bar fraction, but highlight overpredictions for high-mass systems, pointing to potential over-efficiency in central stellar mass build-up in simulations. These findings demonstrate Euclid's transformative potential for galaxy morphology studies and underscore the importance of refining theoretical models to better reproduce observed trends. Future work will explore finer mass bins, environmental correlations, and additional morphological indicators
Euclid Quick Data Release (Q1). The first catalogue of strong-lensing galaxy clusters
No full textInternational audienceWe present the first catalogue of strong lensing galaxy clusters identified in the Euclid Quick Release 1 observations (covering ). This catalogue is the result of the visual inspection of 1260 cluster fields. Each galaxy cluster was ranked with a probability, , based on the number and plausibility of the identified strong lensing features. Specifically, we identified 83 gravitational lenses with , of which 14 have , and clearly exhibiting secure strong lensing features, such as giant tangential and radial arcs, and multiple images. Considering the measured number density of lensing galaxy clusters, approximately for , we predict that \Euclid will likely see more than 4500 strong lensing clusters over the course of the mission. Notably, only three of the identified cluster-scale lenses had been previously observed from space. Thus, \Euclid has provided the first high-resolution imaging for the remaining galaxy cluster lenses, including those with the highest probability. The identified strong lensing features will be used for training deep-learning models for identifying gravitational arcs and multiple images automatically in \Euclid observations. This study confirms the huge potential of \Euclid for finding new strong lensing clusters, enabling exciting new discoveries on the nature of dark matter and dark energy and the study of the high-redshift Universe