Dissertations / Theses on the topic 'Thermonuclear plasmas'

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1990.
Includes bibliographical references (leaves 232-238).
by Emmanouil Antony Chaniotakis.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1990.

ITER (International Thermonuclear Experimental Reactor) rappresenterà un passo fondamentale per la realizzazione della fusione termonucleare controllata. Tra i problemi ancora aperti, il controllo del trasporto turbolento è cruciale per ITER, che richiederà alti valori di temperatura, densità e confinamento del plasma. Questo lavoro si focalizza su quattro aspetti riguardanti il trasporto di calore turbolento nel centro del plasma: l'effetto delle impurezze leggere, l'effetto delle particelle veloci, il ruolo delle instabilità su scala elettronica e delle interazioni multi-scala e l'effetto della massa della specie ionica principale del plasma (effetto isotopico). L'esecuzione e l'analisi di esperimenti in plasmi in L-mode nel tokamak JET è affiancata dall'uso di simulazioni giro-cinetiche con il codice GENE e da test dei modelli quasi-lineari TGLF e QuaLiKiz, usati per la predizione di plasmi di ITER. Il trasporto turbolento è principalmente dovuto a instabilità su scale dell'ordine del raggio di Larmor ionico ed elettronico causate dai gradienti della temperatura ionica ed elettronica oltre una soglia in tali gradienti. Sopra tale soglia, il flusso turbolento cresce con un tasso che determina quanto le temperature sono rigide rispetto a un incremento della potenza di riscaldamento. Soglia e livello di rigidità sono usate in questa tesi per caratterizzare lo stato turbolento del plasma in diverse condizioni. Sono misurate sperimentalmente e comparate con le predizioni numeriche per una validazione dei modelli utilizzati. Le instabilità su scala elettronica sono state trovate fondamentali per spiegare il flusso di calore elettronico sperimentale. Una forte interazione tra scale ioniche ed elettroniche è stata osservata, i modi su scala elettronica essendo più forti quando i modi su scala ionica sono vicini alla stabilità, come sarà in ITER. TGLF è in buon accordo con le simulazioni giro-cinetiche ed è adatto per predizioni qualitative di questi effetti in scenari futuri. Riguardo alle impurezze leggere, le densità di 3He,Be,C,N e Ne e hanno differenti profili radiali negli stessi plasmi, mentre le simulazioni predicono lo stesso profilo per tutte le impurezze. Plasmi in cui N è stato iniettato hanno gradienti di Ti più alti, ben riprodotti e spiegati dalle simulazioni giro-cinetiche. TGLF e QuaLiKiz hanno punti di disaccordo con le simulazioni giro-cinetiche. Indicazioni sui miglioramenti necessari sono state fornite. Una prima prova sperimentale di una forte stabilizzazione del trasporto di calore dovuta alle particelle veloci è stata ottenuta in plasmi con bassa rotazione. Le simulazioni giro-cinetiche indicano due principali meccanismi di stabilizzazione. Uno elettrostatico legato a interazioni risonanti onde-particelle veloci, uno elettromagnetico e sensibile alla pressione del plasma. Anche la funzione di distribuzione delle particelle veloci è determinante. Questi meccanismi non sono ancora stati introdotti nei modelli quasi-lineari. Infine, plasmi di deuterio sono stati comparati con plasmi di idrogeno con simili parametri. Te e Te sono più basse in plasmi di idrogeno e le differenze partono dal bordo del plasma. Nessuna differenza sostanziale è stata osservata nel trasporto di calore a bassa potenza, ma ad alta potenza, quando le particelle veloci sono importanti, il loro effetto di stabilizzazione è minore in plasmi di idrogeno. Una spiegazione è stata trovata nella diversa pressione delle particelle veloci, che in H è la metà di quella in D , a causa di diverse configurazioni dei sistemi di riscaldamento. In conclusione, differenti aspetti del trasporto di calore turbolento sono stati studiati in plasmi L-mode del JET. Un'interpretazione dei risultati sperimentali è stata ottenuta con l'aiuto di simulazioni giro-cinetiche, e alcuni effetti fisici rilevanti per ITER sono stati identificati. Indicazioni sulla validità e su possibili miglioramenti dei modelli utilizzati sono state ottenute.
ITER (International Thermonuclear Experimental Reactor) will represent a fundamental step in the realization of controlled thermonuclear fusion. Among the problems still open, the understanding of the turbulent transport in the plasma is crucial for ITER, that will require high plasma temperature, density and confinement. This work focuses on four topics related to the thermal transport in a tokamak plasma core: the effects of light impurities, the effects of fast particles, the role of electron scale turbulence and multi-scale interactions and the effects of the plasma main ion mass (isotope effect). It covers the execution and analysis of experiments in JET tokamak L-mode plasmas, the use of local gyro-kinetic simulations (using the GENE code) to model the plasma and the test of the quasi-linear models TGLF and QuaLiKiz, used for ITER predictions. The turbulent transport in a tokamak is mainly due to instabilities on scales of the order of ion or electron Larmor radius driven by the ion and electron temperature gradients over a threshold in these gradients. Above the threshold, the turbulent flux increases with a rate that determines how stiff the temperature profiles are against an increase of heating power. Threshold and stiffness are key concepts used in this thesis to characterize the turbulent state of plasmas in different conditions. They are measured experimentally and compared to the theoretical predictions, providing a deeper insight into the plasma behavior and a stringent validation procedure for the models. Electron scale modes have been found determinant to explain the experimental electron heat flux and stiffness. A strong interaction between ion and electron scales was also found, with electron modes being strongest in conditions where ion scale modes are marginal stable, as will be the case in ITER. The TGLF model is in good agreement with the multi-scale gyro-kinetic simulations and can therefore be suitable for at least a qualitative exploration of these effects in future scenarios. Regarding the light impurities, the density profiles of 3He,Be,C,N and Ne show different peaking in the same plasmas, whilst theory predicts similar peaking for all the impurities. Discharges with N injection show higher peaking of Ti, well explained and reproduced by gyro-kinetic simulations. TGLF and QuaLiKiz show several discrepancies with the gyro-kinetic simulations. Indications have been provided on the improvements needed. A first experimental evidence of a strong thermal transport stabilization due to fast ions has been obtained in plasmas with low rotation. Gyro-kinetic simulations indicate two main stabilization mechanisms. One is electrostatic and related to a resonant wave-fast particle interaction, one is electromagnetic and sensitive to the total plasma pressure. The fast ion distribution function has also an influence on the level of the stabilization. These mechanisms are still not included in the quasilinear models. Finally, D plasmas have been compared to H plasmas with similar operational settings. Te and Ti are lower in H plasmas, the difference starting at the plasma edge. No substantial differences have been observed in the thermal transport in the plasma core at low power, but at high power, when fast ions are important, their stabilization effects appear less strong in H plasmas. An explanation has been found in the differences between the fast ion populations, with H plasmas featuring ~1/2 of the fast ion pressure in D, due to the different parameters of the heating systems. In conclusion, different aspects of thermal turbulent transport have been studied in JET L-mode plasmas. An interpretation of the experimental results has been reached with the help of gyro-kinetic simulations, and some physical effects have been evidenced to be relevant for future ITER scenarios. Some important indications on the validity and on possible improvements of the available numerical models have been obtained.

This thesis is aimed at studying the transport of particles in magnetically confined thermonuclear plasma. The understanding of the transport properties in devices for fusion plasmas is one of the key factor to keep the correct operating conditions in a future fusion reactor. Indeed one of the open issues in magnetic fusion studies, which prevents the realization of an efficient thermonuclear reactor, is the high level of energy and particle transport in the direction perpendicular to the confining magnetic field. This phenomenon reduces confinement properties and has to be solved in order to obtain energy from thermonuclear fusion processes. The amount of particle and energy transport experimentally observed cannot be interpreted in the framework of the classical theory. Understanding the underlying physics of this anomalous transport remains the outstanding critical physical issue in fusion research. Nowadays it is generally accepted that anomalous transport is partially due to magnetic chaos owing to the magnetic perturbations of the equilibrium magnetic fields. The Reversed Field Pinch (RFP) configuration, with its wide spectrum of magnetic perturbations, offers a suitable testbed to verify the theory and to reveal the inner mechanism underlying the transport in fusion magnetic devices. The magnetic perturbations, also dubbed dynamo or MagnetoHydroDynamic (MHD) modes, sustain the RFP configuration against the resistive magnetic diffusion. Unfortunately they have global negative effects: as already stated they lead to the stochastization of the equilibrium magnetic field over a large part of the plasma core and moreover their phase locking generates an interference pattern that results in a global distortion of the plasma column: the so-called Locked Mode (LM) that has its maximum effect at a well defined toroidal position. Many techniques have been tested with the aim of reducing the MHD modes. The most effective are the Pulsed Poloidal Current Drive (PPCD) that modifies the internal current profile and the active control of the radial field at the edge by means of a system of active coils, the so-called Virtual Shell (VS). All the transport mechanisms acting inside the plasma modify the shape of the density profile. The density is measured by means of interferometer: a non-perturbative diagnostic that utilizes electromagnetic waves to probe the plasma. A part of this thesis will be addressed to determine the global particle diffusion coefficients in relation to the magnetic perturbations amplitude. This analysis has been carried on TPE-RX device: a large RFP machine sited in Tsukuba (Jp). In order to study the global confinement properties, the transport analysis has been carried out analyzing data collected far from to the LM, where its local effect could be neglected. A transport code (in our case TED, acronym of TEmperature and Density) computes the density profile according to transport parameters supplied by the user. The computed profile is compared to the experimental one, determining the correctness of the model assumed to provide the transport coefficients. With this analysis it has been confirmed that damping the MHD modes amplitude by means of the PPCD the particle confinement globally improves and the diffusion coefficient is strongly reduced in the central zone of the plasma. This result has been further confirmed by the density behaviour during pellet injection experiments, where the particles released by the pellet in PPCD discharges are better confined inside the plasma than in plasmas with standard magnetic perturbations. The dynamo modes, as already stated, generate a global distortion of the Last Close Flux Surface (LCFS) of the plasma: the LM. The plasma cross section results shrunk in a wide toroidal region of about 100° and bulging in another region of the similar toroidal range. Moreover an helical distortion of the column with magnetic lines that directly hit the wall is present. The VS system installed at RFX-mod (the largest RFP device in the world with design maximum plasma current of 2 MA, located in Padova) provides an important reduction of the helical perturbation but is less effective on healing the shrinking of the LCFS, highlighting for the first time its effects on plasma confinement. The two toroidal regions with different cross section have been characterized studying the density profile, the density fluctuations and the magnetic fluctuations: the shrunk region shows an improved transport, providing the first experimental evidence of toroidal asymmetric confinement properties in an RFP plasma. Moreover the RFX-mod pulses are affected by spontaneous reorganization of the internal current and magnetic profiles, the so-called Dynamo Relaxation Events (DREs). The density behaviour and the magnetic topology during the DREs have been analyzed, confirming the different nature of the shrunk and the bulging region of the plasma.

Le comportement d’un plasma de réacteur en fonction de la méthode utilisée pour l’alimenter en particules est difficile à prévoir. Le travail présente ici a été réalisé sur Tore Supra. 2 sujets ont été étudiés : la comparaison du comportement d’un plasma a forte fraction de la densité de Greenwald selon la manière dont il est alimenté et l’étude de l’homogénéisation de la matière déposée par un glaçon (mode d’alimentation prévu pour ITER). Les expériences à forte fraction de Greenwald effectuées sur Tore Supra ont montré que le comportement du plasma dépend de la méthode d'alimentation. Le confinement de l'énergie avec les glaçons est en accord avec les prévisions établies. Ce comportement est moins favorable pour une alimentation par injection supersonique ou classique car une perte de confinement est ici observée. Ce phénomène n'est pas lié au transport mais à la position du dépôt de matière (au bord pour le gaz et au coeur pour les glaçons). Le travail concernant l'homogénéisation de la matière déposée par une injection de glaçon a pour but d'étudier le mouvement de dérive éjectant la matière déposée vers le côté faible champ. Un nouveau phénomène a été mis en évidence : l'influence des surfaces magnétiques à facteur de sécurité (q) entier. Quand la matière dérivant vers le côté faible champ traverse une telle surface, elle subit un phénomène qui arrête la dérive. Ce travail montre aussi que le mouvement de dérive suivant une injection de glaçon côté fort champ est négligeable sur Tore Supra. Cette étude confirme que l'alimentation par injection de glaçons sera un moyen essentiel d'alimentation pour ITER et que l'injection côté faible champ pourrait être reconsidéré
The behaviour of a reactor-class plasma when fuelled using the existing techniques is difficult to foresee. The present work has been initiated on Tore Supra. Two topics have been studied: the comparison of the plasma behaviour when fuelled using the different techniques at high Greenwald density fractions and the study of the homogenisation following a pellet injection (fuelling technique for ITER burning plasmas). The experiments at high Greenwald density fractions performed on Tore Supra showed that the plasma behaviour is dependent on the fuelling method. The plasma energy confinement is following the scaling laws determined at low density when fuelled using pellet injection, which is better than for gas puffing and supersonic injection, both inducing a significant confinement loss. This behaviour is not related to transport but to the position of the matter source (at the edge for gas and close to the centre for pellets). The study concerning the homogenisation phenomena following a pellet injection aims to study the drift effect that expels the mater deposited toward the low field side. A new phenomenon was discovered: the influence of magnetic surfaces with an integer-valued safety factor (q). When the mater drifting toward low field side crosses an integer q surface, it experiences an effect that stops the drift motion. This study allows also determining that the drift following a pellet high field side injection appears negligible in Tore Supra. This work confirms that the pellet injection is an important tool for ITER plasma fuelling and that the low field side injection scheme should not be totally withdrawn for fuelling

This thesis work addresses neutron and γ-ray emission spectroscopy as fast ion diagnostics for fusion plasmas. Two main topics are considered. The first one is the determination of the fast ion energy distribution from measured neutron and γ-ray emission spectra. Neutron spectroscopy has been used at the JET tokamak since 1984. Advanced spectrometers have been built and several features of the neutron emission spectrum have been measured and interpreted in terms of the reaction kinematics. This thesis adds to this knowledge base by inspecting the role of nuclear elastic scattering in producing high energy components in the spectrum from fusion neutrons. The analysis focuses on the possibility to determine the energy distribution tail temperature of radio- frequency heated 3He ions in deuterium plasmas with an admixture of 3He. The RF generation of fast 3He ions is described and the knock-on components are determined with the help of newly derived 3He +d scattering cross sections. Results are presented on the neutron emission spectrum and its contributions from different deuteron velocity components. It is shown that knock-on leaves an observable feature in the spectrum with a clear dependence on absorbed RF power. The investigation is then generalized to D, (3He)D, DT and (3He)DT plasmas, where the relative magnitude of nuclear elastic scattering from different fusion products is determined. The resulting signatures in the neutron emission spectrum are calculated and their relevance for fast ions diagnosis in a burning plasma experiment is discussed. Gamma-ray spectroscopy is a relatively new technique compared to neutron spectroscopy. The Doppler broadening of characteristic γ-ray emission peaks from 12C(3He, pγ)14N reactions in fusion plasmas was measured for the first time in 2008 at the JET tokamak thanks to the installation of a High Purity Germanium detector. In this thesis, intensities and detailed spectral shapes of γ-ray emission peaks are successfully reproduced using a physics model combining the kinetics of the reacting ions with a detailed description of the nuclear reaction differential cross sections for populating the L1-L8 14N excitation levels yielding the observed γ-ray emission. A Monte Carlo code, named GENESIS, was written for the purpose of interpreting γ-ray emission from fusion plasmas and is used here to determine the tail temperature of fast 3He ions from the observed peak shapes. Experiments performed in 4He plasmas of the JET tokamak are also presented. 4He ions were accelerated to the MeV range by coupling third harmonic radio frequency heating to an injected 4He beam. For the first time, Doppler broadening of γ-ray peaks from the 12C(d, pγ)13 C and 9Be(α, nγ)12 C reactions was observed and is here interpreted with the GENESIS code. Acceleration of 4He particles at energies as high as 6 MeV is demonstrated; implications of these results for α particle observations through γ-ray emission spectroscopy in next step deuterium-tritium plasmas are discussed. A second topic addressed by this work is the study of fast ion driven instabilities through γ-ray emission spectroscopy, with emphasis on the development of instrumentation. A high efficiency, high resolution, fast γ-ray spectrometer based on the LaBr3 scintillator and designed for measurements in the MHz range was developed and is here presented. An algorithm based on pulse shape fitting was written to reconstruct γ-ray spectra from digitized data and is shown to provide an energy resolution equivalent to a traditional analog spectrometry chain at low counting rates. The same system was used to perform γ-ray spectroscopy in the MHz range, as demonstrated in experiments performed at nuclear accelerators. γ-ray emission spectra at rates as high as 4 MHz from p + 27Al reactions collected at the Tandem Van der Graaf accelerator of the Nuclear Institute “Horia Hulubei“ (Magurele, RO) are presented and show little or no degradation of the energy resolution. The developed system was also employed to study instabilities driven by fast protons in the ASDEX Upgrade tokamak. The observed γ-ray emission level induced by energetic protons is used to determine an effective tail temperature of the proton distribution function that can be compared with Neutral Particle Analyzer measurements. More generally the measured emission rate is used to assess the confinement of protons with energies less than 400 keV in discharges affected by Toroidal Alfven Eigenmode instabilities. The derived information on confined ions is combined with observations made with the ASDEX Upgrade Fast Ion Loss Detector. The results presented in this thesis represent a step forward in the development of nuclear radiation based methods for burning plasma diagnostics. In particular, they demonstrate that quantitative information on the energy distribution of fast ions and their interaction with plasma instabilities can be inferred from neutron and γ-ray mea- surements by taking into account in detail the reaction processes contributing to the emission of nuclear radiation from the plasma.

Neste trabalho são abordados alguns aspectos do tokamak TCABR, particularmente no que diz respeito à ruptura do plasma, às descargas e ao sistema vertical. Desenvolveu-se um modelo zero-dimensional para modelagem das descargas, envolvendo 5 equações diferenciais. Com esse modelo obteve-se os perfis temporais de parâmetros importantes da descarga tais como: corrente de plasma, tensão de enlace, temperatura eletrônica, densidade eletrônica e densidade de partículas neutras. Verificou-se, com as simulações, a influência de parâmetros importantes no comportamento das descargas. A partir de resultados experimentais do TCA e do TCABR verificou-se a compatibilidade desses resultados com o modelo. Fez-se um estudo da ruptura do plasma no TCABR utilizando-se um modelo físico para os processos que envolvem a ruptura com o qual pode-se delimitar a região de ruptura que depois pode ser comparada com os resultados experimentais. Experimentalmente verificou-se que, para o TCABR, a ruptura ocorre para pressões entre 9.10-6 a 3.10-4 mbar e campos elétricos entre 2 e 10 V/m. A relação campo-pressão, E/p, na região de ruptura, está entre 3.107 e 5.108 V.m-1.bar-1. Foi também feito um estudo do sistema de controle realimentado do campo vertical onde determinou-se algumas funções de transferências importantes, particularmente para os blocos não lineares desse sistema. A partir de um programa computacional foi feito um mapeamento do campo vertical e do índice de curvatura do campo. Verificou-se que a razão entre a componente vertical do campo, no centro do vaso, e o valor correspondente de corrente que passa pelas espiras é de 3,5.10-5 T/A e o índice de curvatura do campo está em torno de 0,45.
In this work, some aspects of the TCABR tokamak are studied. In particular, some points concerned to the plasma breakdown, to discharge characteristics in tokamak mode and to the vertical field system are investigated. A zero-dimensional model has been developed, especially for this work, based on five differential equations involving the ohmic heating circuit and the conservation laws of energy, electrical charge and neutral particles. The model was used for simulating the TCABR plasma discharges. Therefore, time profiles of important plasma parameters like plasma current, loop voltage, electron temperature, electron density and neutral density, were obtained. Also, as a result of the simulations, was verified how the tokamak machine parameters and plasma parameters influence the behavior of the discharges. Some experimental results from the TCABR and TCA were compared with the results of the simulations. A study of the rupture of the plasma was carried out adopting a physical model that includes many physical processes. This model was used to delimit the breakdown region for TCABR tokamak machine and as a tool to understand the experimental data. Experimentally, it was observed that, for TCABR, the rupture occurs for pressures values between 1.10-5 to 3.10-4 mbar, and electric fields values between 2 and 10 V/m. The ratio electric field-pressure (E/p), in the rupture region, is between 3.107 and 5.108 V.m-1.bar-1. For the control system of the vertical field we obtained some transfer functions mainly for the non-linear blocks of the system that have been used in the experimental tests. A computer program was developed to obtain the map of the magnetic vertical field lines and the index of curvature of the field. Finally it was verified that, in the center of the vessel, the ratio between the magnetic vertical field to the electric current that flows in the vertical coils is, 3.5.10-5 T/A and the index of curvature of the field is ~0.45.

This thesis concerns modelling of Coulomb collisions in toroidal plasma with Monte Carlo operators, which is important for many applications such as heating, current drive and collisional transport in fusion plasmas. Collisions relax the distribution functions towards local isotropic ones and transfer power to the background species when they are perturbed e.g. by wave-particle interactions or injected beams. The evolution of the distribution function in phase space, due to the Coulomb scattering on background ions and electrons and the interaction with RF waves, can be obtained by solving a Fokker-Planck equation.The coupling between spatial and velocity coordinates in toroidal plasmas correlates the spatial diffusion with the pitch angle scattering by Coulomb collisions. In many applications the diffusion coefficients go to zero at the boundaries or in a part of the domain, which makes the SDE singular. To solve such SDEs or equivalent diffusion equations with Monte Carlo methods, we have proposed a new method, the hybrid method, as well as an adaptive method, which selects locally the faster method from the drift and diffusion coefficients. The proposed methods significantly reduce the computational efforts and improves the convergence. The radial diffusion changes rapidly when crossing the trapped-passing boundary creating a boundary layer. To solve this problem two methods are proposed. The first one is to use a non-standard drift term in the Monte Carlo equation. The second is to symmetrize the flux across the trapped passing boundary. Because of the coupling between the spatial and velocity coordinates drift terms associated with radial gradients in density, temperature and fraction of the trapped particles appear. In addition an extra drift term has been included to relax the density profile to a prescribed one. A simplified RF-operator in combination with the collision operator has been used to study the relaxation of a heated distribution function. Due to RF-heating the density of thermal ions is reduced by the formation of a high-energy tail in the distribution function. The Coulomb collisions tries to restore the density profile and thus generates an inward diffusion of thermal ions that results in a peaking of the total density profile of resonant ions.

QC 20130415

Cette thèse porte sur l'interaction entre un plasma de bord de tokamak et une perturbation magnétique résonante (RMP), utilisée principalement pour le contrôle de phénomènes de relaxations quasi-périodiques, présents dans un régime de confinement amélioré. Il permet notamment d'atteindre des conditions favorables aux réactions de fusion nucléaire. Il a été observé que la présence de perturbations magnétiques modifie la topologie magnétique au bord ce qui engendre une diminution de l'amplitude des relaxations, voire leur suppression. De précédents travaux ont étudié l'effet de perturbations magnétiques sur un plasma relaxant via des simulations numériques. Le modèle utilisé était dans un cas électrostatique, c'est à dire que la topologie magnétique n'évoluait pas dans le temps. Dans cette thèse, l'étude est faite dans un modèle de plasma de bord prenant en compte les fluctuations magnétiques via le code numérique EMEDGE3D. Ce code a été modifié pour pouvoir imposer une perturbation magnétique résonante. Des vérifications par des modèles réduits ont été menées sur la pénétration d'une perturbation magnétique ainsi que sur l'effet d'une vitesse cisaillée sur la pénétration. Ensuite, un RMP a été imposé dans un plasma non turbulent avec et sans vitesse cisaillée. Un phénomène d'écrantage, empêchant la pénétration d'une perturbation, a été identifié analytiquement et observé dans les simulations. Cette étude a été réitérée dans un plasma turbulent, et aussi en présence d'une barrière (vitesse cisaillée). Le plasma turbulent engendre une amplification du RMP, tandis que la barrière est affectée par la présence de cellules de convection fixes générées par la perturbation
In this thesis, the interaction between tokamak edge plasma and resonant magnetic perturbation (RMPs) is studied. It is mainly used to mitigate quasi-periodic relaxations in enhanced confinement regime. This regime allows to obtain good conditions for nuclear fusion. Introduction of a RMP in a tokamak plasma has been observed to modified the magnetic topology at the edge and decrease the relaxation amplitude up to complete suppression. Previous works studied the RMP effect on a plasma with relaxations, via numerical simulations. The model used for that consider the electrostatic approximation, where the magnetic topology does not evolve in time. In this thesis, the study is done with an edge plasma model taking into account magnetic fluctuations via the numerical code EMEDGE3D. This code has been modified to include the resonant magnetic perturbation. Comparison with reduced models has been carried out on the RMP penetration and the effect of sheared velocity on it. Then a RMP has been induced in a stable plasma, with or without imposed sheared rotation. A condition on the sheared velocity has been identified to avoid the screening effect, that would prevent the RMP penetration, analytically and in numerical simulations. This works has been repeated in a turbulent plasma in presence or not of transport barrier (sheared velocity). The turbulent plasma generate an effective RMP amplification, while the transport barrier is affected by locked convective cells due to the RMP

Plusieurs évènements astrophysiques comme les novae, les supernovae de type Ia (SNeIa) et les sursauts X sont le résultat d'une combustion thermonucléaire explosive dans un plasma stellaire. Les supernovae comptent parmi les objets astrophysiques les plus fascinants tant sur le plan théorique que sur celui des observations. Au moment de l'explosion, la luminosité d'une supernova peut égaler celle de l'intégralité des autres étoiles de la galaxie. On admet aujourd’hui que les SNeIa résultent de l'explosion thermonucléaire d'une étoile naine blanche, un objet dense et compact composé de carbone et d'oxygène. Divers chemins évolutifs peuvent conduire à l’explosion de la naine blanche si celle-ci est membre d’un système stellaire binaire. Néanmoins, la nature du système binaire, les mécanismes d'amorçage et de propagation de la combustion thermonucléaire ainsi que le rapport carbone/oxygène au sein de l'étoile compacte ne sont pas encore clairement identifiés à ce jour. En ce qui concerne l’écoulement réactif, on invoque ainsi une détonation (Modèle sub-Chandrasekhar), une déflagration ou la transition d'une déflagration vers une détonation (Modèle Chandrasekhar). La détonation semble donc jouer un rôle prépondérant dans l'explication des SNeIa.

Les difficultés de modélisation des détonations proviennent essentiellement (i) de la libération d'énergie en plusieurs étapes, de l’apparition d’échelles de temps et de longueurs caractéristiques très différentes (ii) des inhomogénéités de densité, de température et de composition du milieu dans lequel se propage le front réactif et qui donnent naissance aux structures cellulaires et autres instabilités de propagation du front (extinctions et réamorçages locaux).

En plus de celles citées ci-dessus, deux autres difficultés majeures inhérentes à l'étude de ce mode de propagation dans les plasmas stellaires sont rencontrées :la complexité de l’équation d’état astrophysique et la cinétique nucléaire pouvant impliquer plusieurs milliers de nucléides couplés par plusieurs milliers de réactions. Ainsi, les premiers travaux impliquant une combustion thermonucléaire explosive ont été réalisés sur bases d'hypothèses simplificatrices comme l'équilibre nucléaire statistique instantané des produits de réactions ou l'utilisation d'un réseau réduit à une dizaine d'espèces nucléaires. Dans tous ces travaux, la détonation est assimilée à une discontinuité totalement réactive (détonation de Chapman-Jouguet ou CJ). La résolution de l'onde de détonation nécessite l'étude détaillée du processus nucléaire se déroulant dans la zone de réaction. Malheureusement, les supports de calculs actuels ne permettent pas encore ce type de simulations pour les détonations astrophysiques. Le modèle ZND qui constitue une description unidimensionnelle stationnaire de l’écoulement (plan ou courbé) constitue une excellente approximation de la réalité.

Notre travail réexamine les résultats des calculs des structures des ondes de détonations stellaires dans les conditions de température, de densité et de composition envisagées dans les travaux de ce type (détonation CJ et ZND) réalisés jusqu’à présent mais avec une équation d’état appropriée aux plasmas stellaires et une cinétique nucléaire nettement plus riche ;le plus grand réseau jamais utilisé pour ce genre d’études (333 noyaux couplés par 3262 réactions), prenant en compte les données les plus récentes de la physique nucléaire (vitesses de réaction et fonctions de partition)./Several astrophysics events like novae, supernovae and X burts, result from an explosive thermonuclear burning in stellar plasma. Type Ia Supernovae (SNeIa) count amoung the most fascinating stellar objects, they can be more brighter than an entire galaxy. Astrophysic works show that SNeIa may result from a thermonuclear explosion of a compact and dense star called carbon-oxygen white dwarf. The ignition stage and the propagation mode of the thermonuclear combustion wave are not identified yet. The Deflagration-to-Detonation Transition process (or "delayed detonation") sims to give the best overall agrements with the observations :detonations can play appart in SNeIa events.

Simulating thermonuclear detonations count same difficults. The most important are the burning length scales that spent over more than ten oders of magnitud, the nuclear kinetics that involve thousands of nuclids linked by thousands of nuclear reactions and the stellar plasma equation of state (EOS). Hydrodynamical simulations of detonation use very simplified ingedients like reduced reactions network and asymptotic EOS of completely electron degenerate stellar plasma.

Our work is the modelling of these detonations using more representative EOS of the stallar plasma that includs ions, electrons, radiation and electron-pistron pairs. We also use a more

detailed kinetic network, comprising 331 nuclids linked by 3262 capture and photodisintegration reactions, than those usualy employed.

Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Nel programma di ricerca europeo per la fusione termonucleare controllata sono stati definiti otto differenti obiettivi a lungo termine. Una di queste sfide cruciali riguarda lo smaltimento (exhaust) di particelle ed energia provenienti da un reattore a fusione. Per sviluppare e testare delle strategie alternative atte a risolvere il problema dell'exhaust, una nuova macchina sperimentale è attualmente in costruzione in Italia a Frascati presso il centro di ricerca ENEA: il Divertor Tokamak Test facility (DTT). Per progettare un nuovo tokamak sono richiesti sforzi congiunti di fisici ed ingegneri. Al fine di ridurre i costi e minimizzare i rischi, uno strumento essenziale è la modellizzazione integrata il più completa possibile basata su principi primi. Il presente progetto di dottorato è incentrato sullo sviluppo di simulazioni multi-canale basate sulla fisica dei principali scenari operazionali di riferimento di DTT. Modelli all'avanguardia di trasporto, riscaldamento, fuelling ed equilibrio magnetico vengono integrati in queste simulazioni per predire in modo auto-consistente profili di plasma e parametri di scenario. Vengono anche calcolate tutte le interazioni non lineari tra sistemi di riscaldamento e plasma e tra i diversi canali di trasporto. Durante questo lavoro, le simulazioni di DTT sono state progressivamente migliorate perfezionandone le impostazioni e includendo un crescente numero di aspetti grazie all'aggiunta di codici appositi. Inoltre sono stati inclusi man mano aggiornamenti dei sistemi di riscaldamento, dell'equilibrio magnetico e della configurazione della macchina per seguire l'evoluzione del progetto. Il confronto tra simulazioni analoghe con differenti modelli quasi-lineari di trasporto ci rende fiduciosi dell'affidabilità dei profili di plasma predetti e ci permette di identificare i punti deboli dei modelli nei vari regimi in cui opera DTT. Questi modelli quasi-lineari sono stati inoltre validati mediante simulazioni girocinetiche nel range di parametri di DTT. L'accuratezza delle predizioni è state migliorata in modo ricorsivo accordando le condizioni al contorno delle simulazioni di core e delle simulazioni del SOL, garantendo così una consistenza core-edge-SOL. Abbiamo studiato lo scenario a massime performance per guidare la progettazione della macchina e il primo plasma e gli scenari intermedi per assistere le fasi iniziali. Le performance dello scenario a piena potenza è stato testato con nove differenti opzioni di riscaldamento allo scopo di selezionare la distribuzione di potenza ottimale tra i tre sistemi di riscaldamento ausiliario. È stata poi verificata la compatibilità dello scenario a piena potenza con le capacità del sistema di bobine elettromagnetiche. Inoltre per la prima volta sono stati stimati, nello scenario a massima potenza, i denti di sega e gli ELMs di DTT. Un'analisi delle prestazioni richieste ai sistemi di fuelling per sostenere gli alti profili di densità ha dimostrato che sarebbe insufficiente utilizzare solamente un sistema di gas puffing e che sono necessari pellet di deuterio per alimentare DTT. Sono stati stimati i tassi di emissione neutronica, risultando compatibili con il progetto attuale delle schermature neutroniche. Questo progetto di dottorato ha portato all'ottimizzazione delle dimensioni della macchina e alla definizione delle potenze di riferimento dei sistemi di riscaldamento e ha fornito i profili di riferimento per la progettazione delle diagnostiche, la stima delle rese neutroniche, il calcolo delle perdite di particelle veloci, i requisiti del gas puffing e/o dei pellet per il fuelling, valutazioni MHD e altri lavori.
The European research roadmap towards thermonuclear fusion energy defined eight different missions to guide the long–term programme. One of these crucial challenges is the controlled power and particle exhaust from a fusion reactor. To develop and test alternative strategies to solve the exhaust problem, in Italy a new experimental device is now under construction at the ENEA Research Center in Frascati: the Divertor Tokamak Test facility (DTT). Designing a new tokamak requires concerted efforts of physicists and engineers. To reduce costs and minimise risks, a first–principle based integrated modelling as comprehensive as possible of plasma discharges is an essential tool. The focus of this PhD project was to perform the first physics–based multi–channel simulations of the main baseline operational scenarios of DTT. In these simulations state–of–art modules for transport, heating, fuelling, and magnetic equilibrium are integrated to achieve self–consistent predictions of plasma profiles and scenario parameters. All non–linear interactions between heating and plasma and between the different transport channels are also calculated. During this work, the DTT simulations have been progressively enhanced adding codes to include a growing number of aspects and refining run settings. Moreover, updates of the heating systems, magnetic equilibria, and device configuration have been included to comply with the evolving machine design. The comparison among analogous simulations with different quasi–linear transport models made us confident in the reliability of the predicted plasma profiles and allowed us to identify the weak points of the models in the various DTT operational regimes. A validation of these quasi–linear models against the gyrokinetic simulations in the specific DTT range of parameters was also performed. The prediction accuracy has been improved recursively by matching the core and SOL simulation boundary conditions to guarantee the core–edge–SOL consistency. We investigated the full performance scenario to guide the machine design, and the first plasma and intermediate scenarios to assist the commissioning phases. The full performance scenario was tested with nine different heating mix options to select the optimal power distribution amongst the three auxiliary heating systems. The compatibility of the full power scenario with the electromagnetic coil system capabilities was then verified. In addition, the DTT sawteeth and ELMs during the full power scenario were estimated for the first time. An analysis of the required fuelling system performance to sustain the high density profiles proved that only the gas puffing system would be insufficient and that deuterium pellets are needed for the DTT fuelling. Neutron rates were evaluated and found compatible with the present design of the neutron shields. This PhD modelling work led to the optimisation of the device size and of the reference heating mix, and provided reference profiles for diagnostic system design, estimates of neutron yields, calculations of fast particle losses, gas puffing and/or pellet requirements for fuelling, MHD evaluations, and other tasks.

The intensive development of rocket science is attracting more and more attention. The creation of thermonuclear engines will take rocket science to a new level and will make longer flights possible. In the future, the use of rockets with thermonuclear engine could even enable an interstellar voyage to the nearest star system, Alpha Centauri. Nowadays scientists are working on creating weaker and correspondingly more compact reactors that can be installed on the rocket board. The main task of the paper is to describe why the use of thermonuclear engines will lead to more efficient space flight and identify its shortcomings.

Cette thèse porte sur l'estimation simultanée du coefficient de diffusion et du terme source régissant le modèle de transport de la température dans les plasmas chauds. Ce phénomène physique est décrit par une équation différentielle partielle (EDP) linéaire, parabolique du second-ordre et non-homogène, où le coefficient de diffusion est distribué et le coefficient de réaction est constant. Ce travail peut se présenter en deux parties. Dans la première, le problème d'estimation est traité en dimension finie ("Early lumping approach"). Dans la deuxième partie, le problème d'estimation est traité dans le cadre initial de la dimension infinie ("Late lumping approach"). Pour l'estimation en dimension finie, une fois le modèle établi, la formulation de Galerkin et la méthode d'approximation par projection sont choisies pour convertir l'EDP de transport en un système d'état linéaire, temps-variant et à entrées inconnues. Sur le modèle réduit, deux techniques dédiées à l'estimation des entrées inconnues sont choisies pour résoudre le problème. En dimension infinie, l'estimation en-ligne adaptative est adoptée pour apporter des éléments de réponse aux contraintes et limitations dues à la réduction du modèle. Des résultats de simulations sur des données réelles et simulées sont présentées dans ce mémoire.

A partir de l'equation de fokker-planck, nous calculons analytiquement la fonction de distribution d'equilibre des particules thermonucleaires chargees avec des sources et des pertes de particules decrites par des fonctions quelconques. Des expressions explicites et des evaluations numeriques de la fonction de distribution, lorsque les temperatures ionique et electronique sont sensiblement egales, sont donnees pour tous les cas discutes dans la litterature. Nous presentons des calculs numeriques de l'evolution de la fonction de distribution en fonction du temps. Cette fonction de distribution d'equilibre decrit correctement les particules thermonucleaires dans un dispositif de fusion par confinement magnetique ainsi que dans un dispositif de fusion par cibles magnetisees. Nous avons calcule la correction due au champ magnetique de la fonction de distribution des particules du plasma et des particules thermonucleaires chargees. Pour le cas de plasmas magnetises ou le rayon de larmor est beaucoup plus grand que la longueur de debye, l'effet des collisions dans la correction des fonctions de distribution est calcule de facon generale. Nous avons montre que les coefficients de diffusion et de conductivite thermique variaient comme l'inverse du champ dans le calcul du premier ordre et comme l'inverse du carre du champ magnetique dans le calcul du second ordre, en accord avec les resultats publies anterieurement. Cependant, nos calculs donnent une dependance du temps de collisions differente. La correction du premier ordre ne depend pas du temps de collisions et la correction du deuxieme ordre est proportionnelle a l'inverse du temps de collisions. La contribution des particules thermonucleaires a la fonction dielectrique du plasma a ete calculee. Finalement, nous avons effectue des calculs numeriques de l'expression correcte du pouvoir d'arret des particules chargees dans des plasmas magnetises pour des champs magnetiques d'intensite arbitraire.

Į kieto kūno paviršių krentant jonų srautui, jo paviršiuje vyksta procesai, keičiantys medžiagos elementinę sudėtį. Todėl medžiagos paviršiuje susidaro pakeistasis sluoksnis, kuris gali turėti didelę įtaką paviršiaus erozijai. Pavyzdžiui, termobranduolinės energetikos srityje, tokių elementų kaip volframas, anglis ir berilis maišymasis yra svarbūs procesai įtakojantys pirmosios sienelės eroziją, vandenilio išlaikymą ir prasiskverbimą su plazma sąveikaujančioje medžiagoje bei pirmosios sienelės degradaciją. Fizikiniai procesai sukeliantys šiuos pokyčius nėra pilnai suprasti. Darbe pristatomas fenomenologinis plazmos sąveikos su pirmąją sienele modelis. Modelyje atsižvelgiama į keletą galimų vienalaikių plazmos – paviršiaus sąveikos procesų: joninį dulkėjimą, dalelių iš plazmos adsorbciją ant paviršiaus, nudulkėjusių dalelių resorbciją, terminę difuziją, plazmos dalelių implantaciją sienelėje ir jų sukeltą kaskadinį maišymąsi. Taip pat į modelį yra įtraukta pasirinkta cheminė reakcija. Pateikiami modelio taikymo mažų energijų vandenilio plazmos su 0,1 ir 0,8% anglies priemaišų sąveikos su volframo sienele skaičiavimams rezultatai. Gauti modeliavimo rezultatai yra palyginami su eksperimentų duomenimis. Skaičiavimų rezultatai parodo kaskadinio maišymosi svarbą anglies atomų pernašai taikinyje.
If a material is subjected to an ion flux, atomic composition of the material surface layer (first wall) can be modified by various plasma – first wall interaction processes. Therefore, a modified surface layer of the material is formed due to the plasma – first wall interaction. Effects of mixed materials can be a serious concern for the surface erosion. E.g., mixing of materials such as tungsten, carbon and beryllium in the fusion devices can influence important wall parameters such as a wall life time and hydrogen retention and permeation. Physical processes, which cause this modification, are not completely understood. A phenomenological model of plasma – first wall interaction is presented in the paper. Several possible simultaneous processes of plasma – first wall interaction are considered in the model: ion sputtering, adsorption of plasma particles on the surface, resorption of sputtered particles, thermal diffusion, implantation of plasma particles into the wall and cascade mixing. Selected chemical reaction is included in the model as well. Results of the simulation of low energy hydrogen plasma with 0.1% and 0.8% of carbon impurities interaction with tungsten surface are presented. Obtained results of the simulation are compared to the experimental results. Results of the calculation show importance of cascade mixing to the transport of carbon atoms in the target.

This thesis represents the summary of the research activities carried out during a three-years Ph.D. project. The work is divided into two parts, with the common feature of investigating the physical properties related to stability and control of Magneto-Hydro-Dynamic modes in fusion relevant plasmas. One of the aims of the work is to better understand the interaction between such plasmas and a wide range of 3-dimensional electro-magnetic boundary conditions. This part of the research has been carried out on the RFX-mod device, where advanced control strategies have been developed thanks to its state-of-the-art magnetic feedback system. A variety of interlaced problems have been addressed, starting with the improvement of the vacuum magnetic field spectrum through actuator-sensor decoupling, compensation of broken or deactivated coils with simple and real-time applicable strategies and multi-modal Resistive Wall Mode control with varying coil number and geometry. This has allowed to develop relevant control optimization techniques and knowledge for both the Reversed Field Pinch and Tokamak configurations. The former is an excellent playground for RWM studies, given the instability spectrum that is naturally developing. For the latter configuration instead, RWM stability is considered to be one major milestone to be achieved along the road to a commercial fusion reactor. The second part of the work is dedicated to this issue, and deals with the stability properties of Advanced Tokamak scenarios, with reference to the JT-60SA experiment in particular. Studies to understand RWM physics in high beta plasmas, where fluid rotation profiles and hot ions populations from Neutral Beams can play an important role, have been carried out with the MARS-F/K linear MHD codes. If detailed physics such as kinetic effects is coupled to a simplified description of the passive/active structures on one side, on the other hand a simplified plasma can be coupled to a complex 3-D model of the structures to assess realistic active control capabilities of a given system. Different tools are used and described for studying RWM damping physics, and to five a proof-of-principle for feedback control of such instabilities in Advanced Tokamak plasmas operating beyond the no-wall pressure limit.
Questa tesi rappresenta la raccolta delle attività svolte durante i tre anni di un progetto di Dottorato di Ricerca. Il lavoro è stato diviso principalmente in due parti, con il comune denominatore di investigare le problematiche relative alla stabilità e al controllo di instabilità Magneto-Idro-Dinamiche in plasmi di interesse fusionistico. Uno dei principali obiettivi di questo lavoro è lo studio di come questi plasmi interagiscano con diverse condizioni al contorno, strutturali ed elettro-magnetiche, con caratteristiche tridimensionali. Questa parte della ricerca è stata svolta sull'esperimento RFX-mod, dove è stato possibile sviluppare peculiari strategie di controllo grazie all'avanzato sistema di controllo attivo. Sono state affrontate varie problematiche tra loro interconnesse, a partire dallo sviluppo di tecniche per il miglioramento del contenuto armonico dei campi magnetici di vuoto tramite disaccoppiamento attuatori-sensori. Da ciò è stato sviluppato un metodo semplificato e applicabile in tempo reale per la compensazione di attuatori rotti o disattivati, con il medesimo obiettivo di migliorare il contenuto armonico dei campi magnetici prodotti dal sistema di controllo reale. A conclusione di questa parte il controllo multi-modale di modi di parete resistiva (RWM) è stato affrontato, dal punto di vista modellistico e sperimentale. Le strategie sviluppate e gli studi effettuati sono rilevanti sia per la configurazione Reversed Field Pinch sia per il Tokamak. Il primo è un ottimo terreno di prova per studiare i modi RWM, per via dello spettro di instabilità che naturalmente sviluppa. Per la seconda configurazione invece, la stabilizzazione dei modi RWM è considerato uno dei principali obiettivi da raggiungere sulla strada verso un reattore a fusione commerciale. La seconda parte del lavoro è relativa proprio alla problematica della stabilità RWM nella configurazione Tokamak, in particolar modo negli scenari avanzati in fase di sviluppo per l'esperimento JT-60SA. Una serie di studi è stata portata avanti con i codici MARS-F/K per determinare le proprietà dei modi RWM in plasmi ad alto beta, nei quali i profili di rotazione e le popolazioni di ioni sovra termici provenienti dagli iniettori di neutri possono giocare un ruolo importante. Da un lato una descrizione dettagliata del plasma, includendo gli effetti cinetici, è stata accoppiata a un modello semplificato e bidimensionale delle strutture passive. D'altra parte una più semplice descrizione del plasma è stata considerata per l'accoppiamento con un modello dettagliato e tridimensionale delle strutture attive e passive, in quest'ultimo caso è stato possibile sviluppare un modello di controllo attivo in catena chiusa dei modi RWM.

The tokamak is so far the most promising magnetic configuration for achieving a net production of fusion energy. The D-T fusion reactions result in 3.5 MeV alpha-particles, which may destabilize Alfvén eigenmodes through wave-particle interaction. These instabilities redistribute the alpha-particles from the central region of the plasma towards the edge, where they are thermalized, and hence result in a reduced heating efficiency. The high-energy alpha-particles may even be thrown out of the plasma and may damage the wall. To investigate the destabilization of Alfvén eigenmodes by high-energy ions, ion cyclotron resonance heating (ICRH) and neutral beam injection (NBI) are often used to create a high-energy tail on the distribution function. The ICRH does not only produce high-energy anisotropic tails, it also decorrelates the wave-particle interaction with the Alfvén eigenmodes. Without decorrelation of the wave-particle interaction an ion will undergo a superadiabatic oscillation in phase space and there will be no net transfer of energy to the mode. For the thermal ions the decorrelation from collisions dominates while for the high-energy ions the decorrelation from ICRH dominates. As the unstable modes grow up, the gradients in phase space, which drive the mode, are reduced, resulting in a weaker drive. The dynamics of the system becomes non-linear due to a continuous restoration of the gradients by D-T reactions and ICRH. In this thesis the non-linear dynamics of toroidal Alfvén eigenmodes (TAEs) during ICRH has been investigated using the SELFO code. The SELFO code, which calculates the distribution function during ICRH self-consistently using a Monte-Carlo metod, has been upgraded to include interactions with TAEs. The fast decay of the mode amplitude as the ICRH is switched off, which is seen in experiments, as well as the oscillation of the mode amplitude as the distribution function is repetetively built up by the ICRH and flattened by the TAE has been reproduced using numerical simulations. In the presence of several unstable modes the dynamics become more complicated. The redistribution of an alpha-particle slowing down distribution function as well as the reduced heating efficiency in the presence of several modes has also been investigated.
QC 20100628

According to classical linearized resistive magnetohydrodynamics theory, pressuredriven modes are unstable in the reversed-field pinch (RFP) due to unfavorable magnetic field line curvature. The result is based on the assumption of an adiabatic energy equation where anisotropic thermal conduction effects are ignored as compared to convection and compression. In this thesis the effects of heat conduction in the energy equation have been studied. We have examined these effects on the linear stability of pressure-driven resistive modes using boundary value theory (Δ´ ) and a novel initial-value full resistive MHD code employing the Generalized Weighted Residual Method (GWRM). In the Δ´ method, a shooting technique is employed by integrating from the resistive layer to boundaries. The GWRM method, on the other hand, is a time-spectral Galerkin method in which the fully linearized MHD equations are solved. For detailed computations, efficiency requires the temporal and spatial domains to be divided into subdomains. For this purpose, a number of challenging test cases including linearized ideal MHD equations are treated. Numerical and analytical investigations of equilibria reveal that thermal conduction effects are not stabilizing for reactor relevant values of Lundquist number, S0, and normalized pressure, βθ, for tearing-stable plasmas. These studies show that growth rate scales as  γ~_ S0−1/5 , which is weaker than for the adiabatic case, γ~_ S0−1/3. A numerical study of optimized confinement for an advanced RFP scenario including ohmic heating and heat conduction, is also part of this thesis. The fully nonlinear resistive MHD code DEBSP has been employed. We have identified, using both Δ´ and GWRM methods, that the observed crash of the high confinement is caused by resistive, pressure-driven modes.

QC 20130503

Le transport anormal de l’energie observé en régime turbulent joue un rôle majeur dans les propriétés de stabilite des plasmas de fusion par confinement magnétique, dans des machines comme ITER. En effet, la turbulence plasma est intimement corrélée au temps de confinement de l’energie, un point clé des recherches en fusion thermonucléaire.

Du point de vue théorique, la turbulence plasma est décrite par les équations gyrocinétiques, un ensemble d équations aux dérivées partielles non linéaires couplées. Par suite des très différentes échelles spatiales mises en jeu dans des conditions expérimentales réelles, une simulation numérique directe et complète (DNS) de la turbulence gyrocinétique est totalement hors de portée des plus puissants calculateurs actuels, de sorte que démontrer la faisabilité d’une alternative permettant de réduire l’effort numérique est primordiale. En particulier, les simulations de grandes échelles (”Large-Eddy Simulations” - LES) constituent un candidat pertinent pour permettre une telle r éduction. Les techniques LES ont initialement été développées pour les simulations de fluides turbulents à haut nombre de Reynolds. Dans ces simulations, les plus grandes échelles sont explicitement simulées numériquement, alors que l’influence des plus petites est prise en compte via un modèle implémenté dans le code.

Cette thèse présente les premiers développements de techniques LES dans le cadre des équations gyrocinétiques (GyroLES). La modélisation des plus petites échelles est basée sur des bilans d’énergie libre. En effet, l’energie libre joue un rôle important dans la théorie gyrocinétique car elle en est un invariant non lin éaire bien connu. Il est démontré que sa dynamique partage de nombreuses propriétés avec le transfert d’energie dans la turbulence fluide. En particulier, il est montré l’existence d’une cascade d énergie libre, fortement locale et dirigée des grandes échelles vers les petites, dans le plan perpendiculaire â celui du champ magnétique ambiant.

La technique GyroLES est aujourd’hui implantée dans le code GENE et a été testée avec succès pour les instabilités de gradient de température ionique (ITG), connues pour jouer un rôle crucial dans la micro-turbulence gyrocinétique. A l’aide des GyroLES, le spectre du flux de chaleur obtenu dans des simulations à très hautes résolutions est correctement reproduit, et ce avec un gain d’un facteur 20 en termes de coût numérique. Pour ces raisons, les simulations gyrocinétiques GyroLES sont potentiellement un excellent candidat pour réduire l’effort numérique des codes gyrocinétiques actuels.

/ Anomalous transport due to plasma micro-turbulence is known to play an important role in confinement properties of magnetically confined fusion plasma devices such as ITER. Indeed, plasma turbulence is strongly connected to the energy confinement time, a key issue in thermonuclear fusion research. Plasma turbulence is described by the gyrokinetic equations, a set of nonlinear partial differential equations. Due to the various scales characterizing the turbulent fluctuations in realistic experimental conditions, Direct Numerical Simulations (DNS) of gyrokinetic turbulence remain close to the computational limit of current supercomputers, so that any alternative is welcome to decrease the numerical effort. In particular, Large-Eddy Simulations (LES) are a good candidate for such a decrease. LES techniques have been devised for simulating turbulent fluids at high Reynolds number. In these simulations, the large scales are computed explicitly while the influence of the smallest scales is modeled.

In this thesis, we present for the first time the development of the LES for gyrokinetics (GyroLES). The modeling of the smallest scales is based on free energy diagnostics. Indeed, free energy plays an important role in gyrokinetic theory, since it is known to be a nonlinear invariant. It is shown that its dynamics share many properties with the energy transfer in fluid turbulence. In particular, one finds a (strongly) local, forward (from large to small scales) cascade of free energy in the plane perpendicular to the background magnetic field.

The GyroLES technique is implemented in the gyrokinetic code Gene and successfully tested for the ion temperature gradient instability (ITG), since ITG is suspected to play a crucial role in gyrokinetic micro-turbulence. Employing GyroLES, the heat flux spectra obtained from highly resolved direct numerical simulations are recovered. It is shown that the gain of GyroLES runs is 20 in terms of computational time. For this reason, Gyrokinetic Large Eddy Simulations can be considered a serious candidate to reduce the numerical cost of gyrokinetic simulations.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

L'objet de cette these est d'aboutir a une modelisation de l'ensemble des phenomenes de transport observes dans le plasma d'un tokamak que ce soit pour le transport des particules ou celui de l'energie. Pour cela, une methode predictive a ete choisie: la modelisation se traduit en equations mathematiques qui sont ensuite resolues numeriquement de maniere auto-consistante. Cela permet d'ecrire un simulateur de tokamak dont on peut ensuite comparer les predictions aux observations experimentales. Le modele de transport de l'energie est celui de gradient de temperature electronique critique developpe par p-h rebut et al. Tandis que le transport des particules est un travail original presente dans la these. Ce dernier modele met en evidence le lien etroit entre le profil de facteur de securite et le pinch' des particules. L'ensemble de ces deux modeles, complete par une modelisation adequate de la geometrie torique et des termes source, est ensuite compare a une gamme tres large d'observations experimentales (principalement realisees dans le tokamak du jet). En particulier des experiences aussi diverses que les chauffages hors de l'axe, les modes h, les modes a ions chauds ou les plasmas a performances ameliorees suite a une injection de glacon (modes p. E. P. ) sont reproduits avec succes par le simulateur. Ces simulations donnent de solides indications en faveur de l'existence d'un gradient de temperature critique et permettent de comprendre de maniere coherente le transport des particules. Dans un dernier temps les consequences d'un tel modele pour le fonctionnement d'un reacteur de fusion thermonucleaire controlee sont detaillees

La méthode privilégiée d'alimentation des machines à fusion est l'utilisation de glaçons de D et/ou T injectés dans le plasma. Ils sont utilisés actuellement, mais les résultats ne sont pas extrapolables aux futures machines de plus grande taille où le design du système d'injection et la construction de scenarii seront surtout basés sur les simulations. II est donc important de combler les vides dans les modèles existants allant de la fabrication des glaçons au dépôt de matière dans le plasma. Deux manques apparaissent : la modélisation du transport du glaçon dans le tuyau d'injection et la validation du processus d'ablation. Ce travail vise à combler ces vides et comporte 3 parties.- Décrire la physique du dépôt de matière, puis l'état de l'art des principaux résultats et enfin la description des systèmes d'injection de glaçons prévus pour les prochaines machines.- Modéliser le transport du glaçon dans le tuyau d'injection. Les effets pris en compte dans le modèle sont la fragilisation de la glace lors des rebonds, l'augmentation de sa température et son érosion. Le modèle donne notamment le ralentissement et la perte de masse du glaçon au cours du trajet, ainsi que l'énergie élastique stockée lié à son intégrité au sortir du tube.- Contribuer à la validation du code d'ablation HPI2, en comparant ses prédictions aux données mesurées dans les nuages d'ablation. La méthode utilisée est un calcul de jeu de données synthétiques à partir des simulations et en les comparant aux mesures. Cette méthode a permis de valider les hypothèses et approximations du modèle d'ablation susmentionné
The preferred method of fueling fusion device is the use of D and/or T pellets injected into the plasma. They are currently used, but the results cannot be extrapolated to future larger reactors where the design of the injection system and the construction of scenarios will be mainly based on simulations. It is therefore important to fill in the gaps in the existing models from the manufacture of pellets to the deposition of material in the plasma. Two lacks of knowledge appear: the modeling of the pellet transport in the injection pipe and the validation of the ablation process. This work aims to fill these gaps and consists of 3 parts.- Describe the physics of material deposition, then the state of the art of the main results and finally the description of the pellet injection systems planned for the next machines.- Model the transport of the pellet in the injection pipe. The effects taken into account in the model are the weakening of the ice during rebounds, the increase in its temperature and its erosion. The model gives in particular the slowing down and the loss of mass of the pellet during the journey, as well as the stored elastic energy linked to its integrity on leaving the tube.- Contribute to the validation of the HPI2 ablation code, by comparing its predictions to data measured in ablation clouds. The method used is a calculation of synthetic data sets from simulations and comparing them to measurements. This method made it possible to validate the assumptions and approximations of the ablation model

One of the possible ways, how to ensure the necessary electric energy to the future is by the thermonuclear synthesis. Systems that deal with this problem are called stellarator or tokamak. In my work I describe the working principle of the tokamak and its possible future evolution. More precisely I deal with particular tokamak that was brought from Culham to the Prague and its power supply, which became a problem. It is tokamak model COMPASS D. My work consists in power supplying of tokamak reels and power elements in its supply way. However it is just a test gear, which has a long way to the successful end, wich is power supplying instead of power burning. It is just a test gear on which are various tests and analyses carryed out nowadays. Results of these tests will be used for another research and to improve the results of this or other facilities.

Title: Materials for fusion applications and their interaction with tokamak plasma Author: Veronika Klevarová Department: Department of Physics of Materials Supervisor: doc. RNDr. Miloš Janeček, CSc., Department of Physics of Materials Abstract: Tungsten represents a perspective option in the context of fusion devices first-wall materials. In the first part of this work, set of tungsten samples with variable grain size was prepared by spark plasma sintering. Specimens were exposed to steady state deuterium plasma beam and high energy heat pulses, simulating thus the normal operation in the tokamak. As a consequence of the exposure, samples surfaces were roughened, as-prepared grains were recovered and in some cases cracks were formed. Moreover, post-irradiation analysis of the damaged samples revealed activation of in-grain slip systems within the loaded surfaces. Threshold grain diameter for this mechanism was determined to be between 5.5 - 6.6 μm at the particular loading conditions. However, damaged features showed to depend more on the fabrication parameters than on the grain diameter. Synergistic effects of simultaneous loading were proven to be important since those reduced the heat propagation within the volume of the tested samples. In the second part of this thesis, introduction to plasma-surface...