Supraconductivité du composé borocarbure TmNi2B2C (rôle du magnétisme et phase FFLO)

Cette thèse a eu pour objet l'étude de l'influence du magnétisme sur la supraconductivité du composé borocarbure TmNi2B2C. Nous introduisons d'abord le problème de la coexistence de ces deux phénomènes coopératif a priori antagonistes. Nous y situons TmNi2B2C et décrivons également les motivations de ce travail de thèse : TmNi2B2C est un composé où une nouvelle phase supraconductrice doit être favorable, la phase FFLO. Bien que prédite depuis longtemps, une claire mise en évidence expérimentale manque encore. Nous introduisons ensuite plus précisément la classe des borocarbures et TmNi2B2C, pour décrire ensuite nos efforts effectués dans la croissance cristalline de bons échantillons monocristallins. Cette partie se termine sur la description de nos mesures de spectroscopie et de microscopie à effet tunnel. Nous avons mesuré des gaps supraconducteurs très propres et reproductibles, une première sur des supraconducteurs magnétiques. Nous décrivons ensuite le diagramme de phase supraconducteur précis que nous avons obtenu par des mesures de résistivité de TC= 11K jusqu'à 50mK. Nous expliquons sa non-monotonie par un champ critique en limite paramagnétique, à cause du champ d'échange entre les moments localisés de Tm et les électrons supraconducteurs. Des mesures d'aimantation à SQUID, nous permettent d'estimer la force de ce champ d'échange. Enfin, nous décrivons l'influence du magnétisme sur le réseau de vortex, à la recherche d'une signature de la phase FFLO. Nous avons ans un premier temps effectué des mesures de courant critique par transport qui montrent un chute à basse température en opposition avec le comportement habituel. Pour sonder l'état des vortex, nous avons utilisé une technique originale, des mesures de tension transverse. Si le comportement de la tension transverse est celui attendu à haute température (au-dessus de 3K), à basse température, on observe un changement de régime qui peut s'expliquer qualitativement par la présence de la phase FFLO.We present results on the interplay between magnetism and superconductivity in the bororcarbide TmNi2B2C. The bororcarbides are a new family where both magnetism and superconductivity coexist. First, we introduce the question of the coexistence of these two antagonistic phenomena and the aim of this Ph.D.: TmNi2B2C is a compound where a new superconducting phase should be likely to appear. This is the so-called FFLO phase. We wanted to observe this phase in TmNi2B2C. We, then, introduce more precisely the borocarbide family and TmNi2B2C in particular. We show our efforts to provide ourselves with good single crystals. We succeeded to grow good Yni2B2C single crystals, but failed to grow YNi2B2C ones. This second chapter ends with the description of our STM measurements. We have been able to have nice, reproducible gaps, which is a first for magnetic superconductors. The third chapter deals with the study of the influence of magnetism on the critical field. We measured a precise phase diagram down to 50mK with resistivity measurements. We explain its unusual shape by the Pauli limit, enhanced by the exchanged field between localized moments and the superconducting electrons. Magnetization measurements allowed us to get an estimation of the exchange constant. The last chapter deals with the influence of magnetism on the vortices lattice. We first performed some critical currents measurement by transport. The critical current diminished at low temperature though in classical superconductor, it raises. The mechanism for this feature is still to be found. We have probed the state of the vortices lattice through transverse voltage measurements. We show that, although at high temperature, this voltage behaves as expected there is a behavior change under 3K, around the temperature where the FFLO phase is expected. The change in this behavior can be qualitatively explained by the appearance of the FFLO. For a more quantitative description, some precise calculations are needed.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Numerical study of the cryogenic cooling of amplifiers for high power lasers

International audienceThe French collaborative Trio4CLF project aims to understand and control the cryogenic cooling of amplifiers for high power (~1 PetaWatt) and high repetition rate (1-10 Hertz) lasers. In such amplifiers, the fluid (low temperature gaseous helium) evacuates the thermal power absorbed by the solid amplifying plates. A precise knowledge of the heat exchange and of the turbulent fluid flow in the amplifier is requested to evaluate its impact on the laser beam quality. Large Eddy Simulations representative of the amplifier cooling are performed using TrioCFD, a code developed by the CEA. First, validation simulations are carried out for heated periodic channel flows, allowing comparisons with Direct Numerical Simulation results from the literature. They are conducted using conjugate heat transfer calculation between the fluid and the solid. The channel flow is turbulent, with a bulk Reynolds number (based and the bulk velocity and the total height of the channel) of about 14000. Large Eddy Simulation of a heated open turbulent cryogenic helium flow with developing thermal boundary layers is then presented

Equations of state for the thermodynamic properties of binary mixtures for helium-4, neon, and argon

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in the Journal of Physical and Chemical Reference Data and may be found at https://aip.scitation.org/doi/abs/10.1063/1.5142275International audienceBased upon the conceptual design reports for the Future Circular Collider cryogenic system, the need for more accurate thermodynamic property models of cryogenic mixtures of noble gases was identified. Both academic institutes and industries have identified the lack of a reliable equation of state for mixtures used at very low temperatures. Detailed cryogenic architecture modeling and design cannot be carried out without accurate fluid properties. Therefore, the helium-neon equation was the first goal of this work and it was further extended to other fluids beneficial for scientific and industrial applications beyond the particle physics needs. The properties of the noble gas mixtures of helium-neon, neon-argon, and helium-argon are accurately modeled with the equations of state explicit in the Helmholtz energy

Flow and heat transfer around a diamond-shaped cylinder at moderate Reynolds number

International audienceHydrodynamics and heat transfer around a diamond-shaped cylinder in a stationary flow have been investigated using direct numerical simulation. Simulations were carried out for a steady flow with a Reynolds numbers ranging from 1 to 70 and for a Prandtl number corresponding to a gas (). The study focuses on the influence of the diamond apex angle () on the evolution of drag, wake length and Nusselt number. A comparison with the case of a circular cylinder is performed. It is shown that the drag coefficient of a diamond-shaped cylinder remains very close to the one of a circular cylinder () for and that it is reduced by decreasing the apex angle for . In the same time, compared to the circular cylinder case, the reduction of the apex angle postpones significantly the Reynolds corresponding to the wake recirculation onset. When the Reynolds reference velocity is, as often, taken as the far field velocity, the corresponding Nusselt numbers are found to decrease with the apex angle . However, it is found that when the reference velocity is based on the maximal vorticity near the equator, the Nusselt number of diamond-shaped cylinders seems to collapse on a single master curve. This may indicate that the relevant velocity scale to describe Nusselt variation, and thus the heat transfer, is dependant on the interfacial vorticity intensity rather than on the far field velocity

Pressure losses at moderate Reynolds numbers in diamond-shaped cylinders arrays: application to micro-regenerators

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Hydrodynamic experimental and numerical study of micro-fabricated regenerators.

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Simulation des Grandes Échelles d'un écoulement d'air turbulent pour le refroidissement d'amplificateurs de lasers

International audienceThe French collaborative Trio4CLF project aims to understand and control the cryogenic cooling of amplifiers for high power (~1 PetaWatt) and high repetition rate (1-10 Hertz) lasers. In such amplifiers, the fluid evacuates the thermal power absorbed by the solid amplifying plates. A precise knowledge of the heat exchange and thus of the turbulent fluid flow in the amplifier is requested to evaluate its impact on the laser beam quality. As a first step, a Large Eddy Simulation is carried out in air without heating to study the development of the turbulent flow. The CFD code used is TrioCFD, a code developed by the CEA. For validation purpose, the simulation is carried out in the experimental setup configuration: a closed-loop wind tunnel called TRANSAT. Two horizontal plates, separated by 0.05 m, are put in the airflow to represent the amplifier plates. Turbulent boundary layers develop from the plates edges. Numerically, the entrance flow is a homogeneous planar flow with a constant velocity at 10 m/s. The results of this Large Eddy Simulation are presented in this paper as a study of the development of the turbulent boundary layers created by the plates.Le projet collaboratif français Trio4CLF a pour objectif de comprendre et de maitriser le refroidissement cryogénique d'amplificateurs de lasers à haute puissance (~1 PétaWatt) et à haut taux de répétition (1-10 Hertz). Dans de tels amplificateurs, le fluide évacue la puissance thermique déposée dans les plaques amplificatrices solides. Une connaissance fine de l'échange de chaleur et donc de l'écoulement turbulent est nécessaire pour évaluer l'impact du refroidissement sur la qualité du faisceau laser. Dans un premier temps, une Simulation des Grandes Échelles est conduite en air et sans chauffage afin d'étudier le développement de l'écoulement turbulent. Le code de CFD utilisé est TrioCFD, un code développé par le CEA. À des fins de validation, la simulation est menée dans la configuration de l'installation expérimentale : une soufflerie à boucle fermée appelée TRANSAT. Deux plaques séparées de 0,05 m sont placées horizontalement dans l'écoulement d'air de façon à représenter les plaques amplificatrices. Des couches limites turbulentes se développent à partir du bord d'attaque des plaques. Numériquement, l'écoulement d'entrée est un écoulement homogène plan de vitesse constante 10 m/s. Les résultats de cette Simulation des Grandes Échelles sont présentés ici sous la forme de l'étude du développement des couches limites turbulentes créées par les plaques

Thermal design of the SPICA/ESI instrument

The ESI instrument (European SPICA Instrument) is a proposed imaging spectrometer for the 30-210μm band for the JAXA SPICA mission. The instrument will have unprecedented spatial resolution and sensitivity due to the large 03.5m telescope aperture, cold fore-optics (~5K) and high sensitivity detectors (NEP~10-19W/√Hz). One of the key technical challenges of the design of the instrument is the thermal architecture due to the mass and cryogenic heat load constraints and the need for very low temperatures. Two candidate detector technologies have been pre-selected for inclusion in the instrument Phase-A study; Photoconductors and TES Bolometers. An overview of thermal architecture of the SPICA spacecraft is presented in order to explain the thermal interface constraints imposed on the instrument. Proposed thermal architectures for the instrument for both the TES and the Photoconductor options will be outlined including a novel design for a lightweight hybrid cooler for achieving sub 100-mK detector temperatures. This novel cooler architecture utilizes a combination of ADR and sorption coolers. Several design solutions for achieving high thermal isolation generic to both detector options are presented