<p>The topic of this thesis concerns the LHC, the next large particle accelerator at CERN which will start operating in 2007. Being based on superconductivity, the LHC needs to operate at very low temperatures, which makes great demands on the cryogenic system of the accelerator. To cope with the heat loads induced by the particle beam, a beam screen cooled with forced flow of supercritical helium is used.</p><p>There is an interest in upgrading the energy and luminosity of the LHC in the future and this would require a higher heat load to be extracted by the beam screen cooling system. The objective of this thesis is to quantify different ways to upgrade this system by mainly studying the effects of different pressure and temperatures levels as well as a different cooling medium, neon.</p><p>For this a numerical program which simulates one-dimensional pipe flow was constructed. The frictional forces were accounted for by the empirical concept of friction factor. For the fluid properties, software using empirically made correlations was used. To validate the numerical program, a comparison with previous experimental work was done. The agreement with experimental data was good for certain flow configurations, worse for others. From this it was concluded that further comparisons with experimental data must be made in order to tell the accuracy of the mathematical model and the correlations for fluid properties used.</p><p>When using supercritical helium, thermo-hydraulic instabilities may arise in the cooling loop. It was of special interest to see how well a numerical program could simulate and predict this phenomenon. It was found that the numerical program did not function for such unstable conditions; in fact it was much more sensitive than what reality is.</p><p>For the beam screen cooling system we conclude that to cope with the increased heat loads of future upgrades, an increase in pressure level is needed regardless if the coolant remains helium, or is changed to neon. Increasing the pressure level also makes that the problems with thermo-hydraulic instabilities can be avoided. Of the two coolants, helium gave the best heat extraction capacity. Unlike neon, it is also possible to keep the present temperature level when using helium.</p>

The topic of this thesis concerns the LHC, the next large particle accelerator at CERN which will start operating in 2007. Being based on superconductivity, the LHC needs to operate at very low temperatures, which makes great demands on the cryogenic system of the accelerator. To cope with the heat loads induced by the particle beam, a beam screen cooled with forced flow of supercritical helium is used. There is an interest in upgrading the energy and luminosity of the LHC in the future and this would require a higher heat load to be extracted by the beam screen cooling system. The objective of this thesis is to quantify different ways to upgrade this system by mainly studying the effects of different pressure and temperatures levels as well as a different cooling medium, neon. For this a numerical program which simulates one-dimensional pipe flow was constructed. The frictional forces were accounted for by the empirical concept of friction factor. For the fluid properties, software using empirically made correlations was used. To validate the numerical program, a comparison with previous experimental work was done. The agreement with experimental data was good for certain flow configurations, worse for others. From this it was concluded that further comparisons with experimental data must be made in order to tell the accuracy of the mathematical model and the correlations for fluid properties used. When using supercritical helium, thermo-hydraulic instabilities may arise in the cooling loop. It was of special interest to see how well a numerical program could simulate and predict this phenomenon. It was found that the numerical program did not function for such unstable conditions; in fact it was much more sensitive than what reality is. For the beam screen cooling system we conclude that to cope with the increased heat loads of future upgrades, an increase in pressure level is needed regardless if the coolant remains helium, or is changed to neon. Increasing the pressure level also makes that the problems with thermo-hydraulic instabilities can be avoided. Of the two coolants, helium gave the best heat extraction capacity. Unlike neon, it is also possible to keep the present temperature level when using helium.

Au cours des dernières décennies, le phénomène de quench a été une des problématiques les plus importantes abordées dans les conceptions d’aimants supraconducteurs. En effet, la transition de quench d’un aimant de son état supraconducteur à son état normal induit une grande quantité de l’énergie par effet Joule. Cet apport de chaleur va ensuite augmenter rapidement la température du conducteur ainsi que la pression du liquide de refroidissement à l’hélium. Le dépassement d’un certain seuil sur ces deux paramètres peut engendrer une détérioration irréversible à l’aimant et au système de refroidissement cryogénique. Afin de mettre en évidence les comportements de quench des bobines supraconductrices à champ toroïdal (TF) du Tokamak JT-60SA, nous avons réalisé des études expérimentales et numériques sur les phénomènes thermohydrauliques résultant du quench d’un aimant supraconducteur fabriqué en câble-en-conduit conducteur (CICC) et refroidi par l’écoulement forcé à l’hélium supercritique. Dans ce cadre, toutes les 18 TF bobines de JT-60SA ont été testées dans une configuration à une seule bobine dans leurs conditions de fonctionnement nominales de courant et de température (25,7 kA et 5 K). Une augmentation progressive de la température a été appliquée à l'entrée de l'hélium jusqu'à la température de quench, suivie d'une décharge rapide du courant dès que le quench est détecté pour protéger l'aimant. Les analyses expérimentales de ces tests ont d'abord permis d'identifier plusieurs phases dynamiques très différentes pendant toute la propagation de quench. Ensuite, les phénomènes physiques parcourant chacune de ces phases ont été étudiés et les plus prédominants ont été mis en évidence tels que les charges thermiques externes, les performances magnétiques des brins, les transferts thermiques conducto-convectifs entre conducteurs et hélium ou encore l'expulsion d'hélium et le reverse flow. Sur la base de ces analyses expérimentales, un modèle numérique d’une seule galette a été développé dans le code THEA afin d'analyser un phénomène physique à la fois sans construire un modèle global trop complexe de l'ensemble de l'aimant. Ce modèle d’une seule galette a été validé sur les données d'expériences de quench et a été appliqué avec succès pour faire d'autres analyses plus détaillées des phénomènes physiques ainsi que des phases dynamiques identifiées pendant la propagation de quench des TF bobines. Ce modèle numérique a même permis d'identifier certains phénomènes prépondérants qui n'ont pas pu être étudiés dans l'analyse expérimentale, tels que l'impact des instabilités des conditions de test sur la dynamique de quench. Les très bons résultats de ce modèle et sa cohérence avec les analyses physiques expérimentales en font une étape très intéressante vers la modélisation complète de toute la TF bobine de JT-60SA et l'étude de son comportement de quench dans une vraie machine Tokamak et non en conditions d'essais. / During the last decades, the quench phenomenon has been one of the most important issues addressed in the superconducting magnets designs. Indeed, the quench transition of a magnet from its superconducting state to its normal state induces a large deposition of the Joule effect energy leading to an abrupt temperature increase in the conductor as well as a large pressure rise in the helium coolant. Any excess of these two parameters can cause an irreversible damage either to the magnet or to the cryogenic system. In order to achieve a better understanding of the quench behavior of the TF coils in the superconducting Tokamak JT-60SA, we carried out both experimental and numerical studies of the thermohydraulic phenomena taking place during the quench of a superconducting magnet manufactured with Cable-In-Conduit Conductor and cooled in forced flow with supercritical helium. In this framework, all the 18 JT-60SA TF coils were tested in a single coil configuration at their nominal operating conditions of current and temperature (25.7kA and 5K). A progressive temperature increase has been applied to the helium inlet up to the quench temperature, followed by a current fast discharge as soon as the quench is detected to protect the magnet. The experimental analyses of these tests allowed first to identify several very different dynamic phases in the overall quench propagation time. Then, the physical phenomena driving each one of these phases have been studied and the most predominant ones have been highlighted such as the external heat loads, the strands magnetic performances, the conductive and convective heat transfers between conductors and helium or even the helium expulsion and reverse flow. Based on these experimental analyses, a single pancake numerical model has been developed in the THEA code in order to analyze one physical phenomenon at a time without building a too complex global model of the entire magnet. This single pancake model has been validated on the quench experiments data and has been successfully applied to make further more detailed analyses of the physical phenomena as well as the dynamic phases identified during the TF coils quench propagation. This numerical model even allowed identifying some driving physical phenomena that could not be studied in the experimental analysis, such as the impact of the testing conditions instabilities on the quench dynamics. The very good results of this model and its coherence with physical experimental analyses makes it a very interesting step towards the full modelling of the entire JT-60SA TF coil and the study of its quench behavior in real Tokamak and not test facility conditions.