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Concept# Énergie de fusion nucléaire

Résumé

vignette| L'expérience de fusion magnétique du Joint European Torus (JET) en 1991.
L'énergie de fusion nucléaire est une forme de production d'électricité du futur qui utilise la chaleur produite par des réactions de fusion nucléaire. Dans un processus de fusion, deux noyaux atomiques légers se combinent pour former un noyau plus lourd, tout en libérant de l'énergie. De telles réactions se produisent en permanence au sein des étoiles. Les dispositifs conçus pour exploiter cette énergie sont connus sous le nom de réacteurs à fusion nucléaire. La recherche sur les réacteurs à fusion a commencé dans les années 1940, mais à ce jour, en 2023, un seul type de conception, une machine à fusion par confinement inertiel au National Ignition Facility (NIF) aux USA, a en tout et pour tout produit un facteur de gain d'énergie de fusion supérieur à 1, c'est-à-dire que les réactions de fusion ont produit une quantité d'énergie supérieure à celle qu'il a fallu fournir au combustible pour maintenir les

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Tokamak

thumb|Vue intérieure du tore du Tokamak à configuration variable (TCV), dont les parois sont recouvertes de tuiles de graphite.
Un tokamak est un dispositif de confinement magnétique expérimental ex

ITER

Le réacteur thermonucléaire expérimental international, ou ITER (acronyme de l'anglais International thermonuclear experimental reactor, également mot latin signifiant « chemin » ou « voie »), est un

Fusion nucléaire

vignette|Le Soleil est une étoile de la séquence principale, dont l'énergie provient de la fusion nucléaire de noyaux d'hydrogène en hélium. En son cœur, le Soleil fusionne de tonnes d'hydrogène chaq

Cours associés (31)

PHYS-424: Plasma II

This course completes the knowledge in plasma physics that students have acquired in the previous two courses, with a discussion of different applications, in the fields of magnetic confinement and controlled fusion, astrophysical and space plasmas, and societal and industrial applications.

PHYS-445: Nuclear fusion and plasma physics

The goal of the course is to provide the physics and technology basis for controlled fusion research, from the main elements of plasma physics to the reactor concepts.

PHYS-439: Introduction to astroparticle physics

We present the role of particle physics in cosmology and in the description of astrophysical phenomena. We also present the methods and technologies for the observation of cosmic particles.

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State-of-the-art magnets of fusion devices, which are based on low temperature superconductors (LTS), have almost reached their technological limits in terms of generated magnetic fields. Further progress can be made using novel high temperature superconductors (HTS). Although HTS have been discovered several decades ago and in present time are ready for the long-length production, the feasibility of HTS cables for fusion magnets is not yet demonstrated. The main challenge toward HTS fusion cables is posed by essentially different geometry of conductors. While LTS conductors can be manufactured in a favorable geometry of round wires, the most promising HTS materials are only available as thin tapes. Thus, new cabling concepts suitable to arrange hundreds of thin tapes should be identified and developed. In this research we aim at demonstrating experimentally the applicability of HTS materials for fusion magnets. This task starts from the development of an intermediate cabling 'solution' -the strand- where a stack of ten to forty tapes is encased between two semicircular copper profiles. Then, the components are twisted and soldered together. Investigations of the strand electrical and electromechanical properties are of key importance to assess the potential application to fusion magnets. Thanks to its round geometry, the HTS strand can be used in conventional cabling methods. The Rutherford-like cable design, with the inclusion of central copper core, is investigated for use with the round strands. As a result of full-scale R & D activity at the Swiss Plasma Center, 60 kA-class HTS cable prototypes have been manufactured and successfully tested at low temperatures, from 5 K to 40 K, and high background magnetic fields, up to 12 T. Results and implications of these measurements constitute the backbone of this thesis. The AC loss and quench properties of the proposed cable design are also studied. Experimental data for the AC loss have separately been acquired on the tape, strand and cable stages at operating conditions relevant for fusion magnets. This allowed us to validate the numerical tools developed to assess the hysteresis loss in stacks and coupling current losses in the strand and cable. The quench studies -mostly the protection issues- are also addressed in this thesis via numerical modeling. Application of the HTS cables to fusion magnets leads to various improvements in the magnet system such as an access to magnetic fields above 15 T limit and increased temperature margins at the cable operation. When applied to the central solenoid, the use of the HTS cables may either reduce its dimensions at a given generated magnetic flux, or increase the flux when the dimensions are kept fixed. Although presently the price of HTS materials is still at least 5 to 10 times higher than of 'classic' LTS materials, use of HTS at high fields is fully justified being the only available option for such application. Manufacturers of the HTS materials keep improving the price to performance ratio of their product, which also increases the chance that the HTS conductor technology can soon be used for fusion magnets.

The transport of particles in magnetically confined plasmas is of great importance for the development of fusion energy. It will determine techniques for fuelling, for controlling impurity concentrations and for the removal of the alpha particles produced by fusion reactions. The issues related to particle transport have received, until relatively recently, less attention than those regarding heat transport. Besides the greater experimental difficulty of measuring particle transport, this may explain why our understanding of this subject is still incomplete. The aim of this thesis is to document and tentatively interpret the experimental density profile behaviour in the TCV (Tokamak à Configuration Variable) and JET (Joint European Torus) tokamaks in the framework of different theoretical and semi-empirical models. The TCV tokamak is well suited to transport studies due to its extreme shaping capability, which allows the exploration of a wide range of different plasma conditions. This versatility is matched by the powerful and flexible electron cyclotron heating (ECH) system on TCV, which allows a control of the local power deposition profiles and current drive profiles. A study of particle transport on the JET tokamak has allowed us to compare the results of TCV to those of a much larger device and supplement the TCV study with the analysis in reactor relevant high confinement regime (H-mode). The experimental information was compiled into a database of density profiles in steady state, containing nearly 1000 samples for TCV and 600 samples for JET. The data analyzed covered a wide range of discharge conditions, including low confinement regime (L-mode) and H-mode discharges, ECH, ICH (ion cyclotron heating), LHCD (low hybrid), beam heated plasmas and include fully current drive discharges. The most relevant parameters which influence the density profiles were determined by regression. A detailed analysis of the particle sources showed that edge fuelling in TCV and JET cannot be responsible for density gradient in the plasma bulk, confirming the presence of particle convection or a 'pinch'. The existence of an anomalous pinch was unambiguously demonstrated both on JET and on TCV by the observation of peaked density profiles in stationary, fully relaxed, fully current driven discharges and hence in the absence of the neoclassical Ware pinch. An unexpected difference in the parameter dependencies was found in L-and H-modes. In TCV and JET, density gradient lengths (or profile peaking parameters) in L-mode were found to depend on magnetic shear with no dependence on collisionality. This lack of collisionality dependence in L- mode is inconsistent with current theoretical models. H-mode density profiles in JET, on the other hand, are clearly dependent on collisionality in agreement with theory and a prior observation on ASDEX-Upgrade, while exhibiting only a weak or no dependence on shear and temperature profiles. It was found that for TCV and JET, L mode density peaking can be interpreted as being due to turbulent equipartition, which assumes conservation of the magnetic moment and the longitudinal invariant during transport. The observation of a reduction of peaking in TCV with ECH supports drift wave turbulence theory, which predicts the appearance of outward particle convection, when trapped electron modes are destabilized. In JET H-mode, the weak secondary correlation of peaking with the electron-ion temperature ratio Te/Ti, may also be considered, at least quantitatively, as being supportive of drift wave turbulence theory.

In magnetic confinement devices, the inhomogeneity of the confining magnetic field along a magnetic field line generates the trapping of particles (with low ratio of parallel to perpendicular velocities) within local magnetic wells. One of the consequences of the trapped particles is the generation of a current, known as the bootstrap current (BC), whose direction depends on the nature of the magnetic trapping. The BC provides an extra contribution to the poloidal component of the confining magnetic field. The variation of the poloidal component produces the alteration of the winding of the magnetic field lines around the flux surfaces quantified by the rotational transform ι. When ι reaches low rational values, it can trigger the generation of ideal MHD instabilities. Therefore, the BC may be responsible for the destabilisation of the configuration. This thesis is divided into two parts. In the first part, we present a self-consistent method to calculate the BC and assess its effect on equilibrium and stability in general 3D configurations. This procedure is applied to two reactor-size prototypes (both with plasma volumes ∼ 1000m3): a quasi-axisymmetric (QAS) system and a quasi helically symmetric (QHS) system with magnetic structures that develop BC in opposite directions. The BC increases with the plasma pressure, therefore its relevance is enhanced when dealing with reactor-level scenarios. The behaviour of both prototypes at reactor level values of β ≡ (kinetic plasma pressure)/(magnetic pressure) is assessed, as well as its alteration of the equilibrium and stability. In the QAS prototype, BC-consistent equilibria have been computed up to β = 6.7% and the configuration is shown to be stable up to β = 6.4%. Convergence of self-consistent BC calculations for the QHS case is achieved only up to β = 3.5%, but the configuration is unstable for β ≥ 0.6%. The relevance of symmetry breaking modes of the Fourier expansion of the confining magnetic field on the generation of BC is studied for each prototype. This proves the close relationship between magnetic structure and BC. Having established the potentially dangerous implication of the BC, principally, in reactor prototypes, a method to compensate its harmful effects is proposed in the second part of the thesis. It consists of the modelling of the current driven by externally launched ECWs within the plasma to compensate the effects of the BC. This method is flexible enough to allow the identification of the appropriate scenarios in which to generate the required CD depending on the nature of the confining magnetic field and the specific plasma parameters of the configuration. Both the BC and the CD calculations are included in a self-consistent scheme which leads to the computation of a stable BC+CD-consistent MHD equilibrium. This procedure is applied in this thesis to simulate the required CD to stabilise the QAS and QHS prototypes introduced in the first part. The estimation of the input power required and the effect of the driven current on the final equilibrium of the system is performed for several relevant scenarios and wave polarisations providing various options of stabilising driven currents. Several scenarios have been devised for each prototype in order to drive current at the appropriate location and with the desired direction. Different polarisations and launching conditions have been employed to this purpose. In particular, a HFS launched X2 ECW with an input power of 1.5MW has been shown to drive sufficient current to maintain the rotational transform below the critical value 2/3 at β = 6.4% for the QAS reactor. Correspondingly, in the QHS reactor, an X3-mode ECW of 100KW was sufficient to drive the current required to push the rotational transform below unity near the magnetic axis at β = 3%. Thus, stabilisation of BC-driven instabilities with externally launched ECWs has been achieved for both contrasting configurations. The method proposed in this thesis allows also the utilisation of EBW in the generation of CD. The possible advantages of EBCD for compensation studies are described as well as their possible application to the two prototypes under consideration. The BC+CD procedure is particularly interesting to investigate new magnetic geometries as potential candidates for fusion reactors. With this numerical tool, it is possible to assess the implications of their consistent BC when operating at reactor level. It also allows to quantify how much power would be required to maintain the system MHD stable in these circumstances. Nevertheless, this method is flexible enough to be applicable to any configuration.