Author: Alessandro Casati
Publisher:
ISBN:
Category :
Languages : en
Pages : 121
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Book Description
Le développement d'un modèle de transport gyrocinétique quasi-linéaire pour les plasmas de tokamak, conçu pour fournir des prévisions physiquement fiables des quantités thermodynamiques pertinentes, est une tâche qui a exigé des liens étroits entre les études théoriques, expérimentales et numériques. Le cadre du modèle ici développé, Qualikiz, qui exploite une réduction de complexité par rapport à la dynamique non-linéaire du plasma, permet de multiples validations de la compréhension actuelle de la micro-turbulence dans les tokamaks. Les principaux résultats de cette thèse découlent des étapes fondamentales de la formulation du modèle de transport quasi-linéaire, c'est-à-dire : (1) la vérification de la réponse quasi-linéaire contre les résultats numériques non-linéaires, (2) l'amélioration du modèle de la saturation grâce à une validation quantitative des codes non-linéaires contre les mesures de turbulence, (3) l'intégration du modèle quasi-linéaire dans un solveur de transport intégré.
Author: Pierre Cottier
Publisher: LAP Lambert Academic Publishing
ISBN: 9783659411038
Category :
Languages : en
Pages : 128
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Book Description
The magnetic confinement in tokamaks is for now the most advanced way towards energy production by nuclear fusion. Both theoretical and experimental studies showed that rotation generation can increase its performance by reducing the turbulent transport in tokamak plasmas. The rotation influence on the heat and particle fluxes is studied along with the angular momentum transport with the quasi-linear gyro-kinetic eigenvalue code QuaLiKiz. For this purpose, the QuaLiKiz code is modified in order to take the plasma rotation into account and compute the angular momentum flux. It is shown that QuaLiKiz framework is able to correctly predict the angular momentum flux including the ExB shear induced residual stress as well as the influence of rotation on the heat and particle fluxes. The different contributions to the turbulent momentum flux are studied and successfully compared against both non-linear gyro-kinetic simulations and experimental data.
Author: Pierre Cottier
Publisher:
ISBN:
Category :
Languages : en
Pages : 118
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Book Description
Author: Cole Darin Stephens
Publisher:
ISBN:
Category :
Languages : en
Pages : 312
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Book Description
The quest to harness fusion energy requires the successful modeling of plasma turbulence and transport in magnetic confinement devices. For such modeling, the requisite length and time scales span many orders of magnitude. Integrated modeling approaches are constructed to account for the wide range of physics involved in turbulent transport by coupling separate physical models together. The primary physical models used in this work are kinetic and designed to simulate microturbulence on the smallest scales associated with turbulent transport. However, high precision nonlinear kinetic simulations often cannot be easily coupled to integrated modeling suites due to the extreme computational costs that would be involved. Model reduction which drastically reduces the computational complexity of the problem is therefore necessary. One must of course ensure that the reduced model does not severely diminish the accuracy of the calculation; the model reduction itself must be founded on more exact computational approaches as well as fundamental theoretical principles. One of the most successful approaches in model reduction is quasilinear gyrokinetics. There are two fundamental assumptions for the quasilinear model examined in this work. First, the three adiabatic invariants (the magnetic moment, the longitudinal invariant, and the poloidal flux) must be appropriately conserved and their associated single charged particle motions (the gyromotion, the bounce-transit motion, and the toroidal drift motion) must be characterized accurately. Second, the quasilinear approximation must hold such that the coherent linear response is adequate enough to compute the quasilinear fluxes without full calculation of the nonlinear physics. The particular model used, QuaLiKiz, has been proven successful in reproducing local gyrokinetic fluxes in the tokamak core while remaining computationally tractable. There are three primary goals of this dissertation project. The first is to examine the fundamental physics underlying gyrokinetic and reduced model approaches at the single charged particle scale. To achieve this goal, we examine the assumption of magnetic moment invariance in a wide variety of electromagnetic fields. We successfully identify the dimensionless parameters that determine magnetic moment conservation in each scenario and then proceed to quantify the degree to which magnetic moment conservation is broken. In doing so, we confirm that the magnetic moment is sufficiently conserved for a wide range of regimes relevant to tokamak plasmas. In addition, we derive new analytic formulas for quantities associated with bounce-transit motion in circular tokamak fields. We compare these new, more exact calculations to approximations commonly used in reduced models (including QuaLiKiz) and determine the conditions such that the approximations break down. We then also confirm that the approximations are valid in the tokamak core for conventional, large aspect ratio devices. The second goal of this dissertation project is to rederive and compile the model equations for QuaLiKiz from first principles. Over the years of QuaLiKiz's development, there has never been a complete manuscript that sketches the derivation of QuaLiKiz from start to finish. The lack of such a document makes it difficult to extend the physics of QuaLiKiz to new parameter regimes of interest. Various possible extensions such as including electromagnetic effects or more realistic tokamak geometries require the adjustment of several different assumptions that would affect the derivation in key ways. As such, correct implementations of new physics would require an existing derivation as a reference point lest the implementation be handled in an incoherent fashion. In addition, a step-by-step outline of how each assumption of QuaLiKiz affects the derivation can be helpful in determining which assumptions can be relaxed for a more accurate model. The successful completion of this derivation, included in this dissertation, will be immensely useful for future QuaLiKiz improvement and validation. With the derivation in hand, we proceed to the third goal of this project: improving the collisional model of QuaLiKiz. Collisions play an essential role in characterizing the transport associated with trapped electron modes. It has become evident in recent studies that the collisional model in QuaLiKiz requires improvement; in integrated modeling, the imprecise treatment of collisional trapped electron modes leads to incorrect density profile predictions near the tokamak core for highly collisional regimes. We revisit the collision model implemented in QuaLiKiz and use the more exact gyrokinetic code GENE (Gyrokinetic Electromagnetic Numerical Experiment) to make improvements to QuaLiKiz's collision operator. We then use the new version of QuaLiKiz in integrated modeling to compare density profiles predicted by the old and new collision operators. We confirm that the new collision operator leads to density profiles that more accurately match the experimental profiles.
Author: Camille Gillot
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Optimal control of tokamak plasmas requires efficient and accurate prediction of heat and matter transport. Growing from kinetic resonant instabilities, turbulence saturates by involving many scales, from the small vortex up to the back-reaction on the density and temperature profiles. Self-organisation processes are of particular interest, encompassing spontaneous zonal flow genera- tion and transport by avalanche. First principle numerical simulation codes like GYSELA allow studying the gyro-kinetic evolution of the particle distribution function. The large model size and cost prompts the need for reduction. Removing velocity dimensions is the so-called collisionless closure problem for fluid equations. Earlier approaches are extended and generalised by calling to the dynamical systems and optimal control litterature. In particular, we apply the balanced truncation and rational interpolation to the one-dimensional linear VlasovPoisson problem. The interpolation method features a cheap and versatile formulation, opening the door to wider use for more complex phenomena. Quasi-linear theory is the reference model for turbulent effects. The GYSELA three-dimensional output is analysed to estimate the robustness of linear properties in turbulent filaments. Key quasi-linear quantities carry over to the non-linear regime. Effective velocities and shape of turbulent structures are computed, and match expected group velocities and linear eigenmode. Nevertheless, the turbulent potential spectrum must be specified externally to quasi- linear models. This results in radially travelling unstable linear solutions that share many properties of turbulent avalanches seen in numerical simulations.
Author: Radu Balescu
Publisher: CRC Press
ISBN: 9781420034684
Category : Science
Languages : en
Pages : 498
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Book Description
Anomalous transport is a ubiquitous phenomenon in astrophysical, geophysical and laboratory plasmas; and is a key topic in controlled nuclear fusion research. Despite its fundamental importance and ongoing research interest, a full understanding of anomalous transport in plasmas is still incomplete, due to the complexity of the nonlinear phenomena involved. Aspects in Anomalous Transport in Plasmas is the first book to systematically consider anomalous plasma transport theory and provides a unification of the many theoretical models by emphasizing interrelations between seemingly different methodologies. It is not intended as a catalogue of the vast number of plasma instabilities leading to anomalous transport; instead it chooses a number of these and emphasizes the aspects specifically due to turbulence. After a brief introduction, the microscopic theory of turbulence is discussed, including quasilinear theory and various aspects of renormalization methods, which leads to an understanding of resonance broadening, mode coupling, trajectory correlation and clumps. The second half of the book is devoted to stochiastic tramsport, using methods based on the Langevin equations and on Random Walk theory. This treatment aims at going beyond the traditional limits of weak turbulence, by introducing the recently developed method of decorrelation trajectories, and its application to electrostatic turbulence, magnetic turbulence and zonal flow generation. The final chapter includes very recent work on the nonlocal transport phenomenon.
Author: Emil Fransson
Publisher:
ISBN: 9789179058395
Category : Magnetohydrodynamics
Languages : en
Pages : 0
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Book Description
Author: Andreas Skyman
Publisher:
ISBN: 9789173859615
Category :
Languages : en
Pages :
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Book Description
Author: Daniel Tegnered
Publisher:
ISBN: 9789175975603
Category :
Languages : en
Pages :
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Book Description
Author: Julien Medina
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
One of the most promising approach to controlled nuclear fusion is the tokamak. It is a toroidal machine confining a fusion plasma using magnetic fields. Transport of particles and heat, from the core toward the edges happens spontaneously, degrades the efficiency of the tokamak, and is driven by turbulence. We use a bounce-averaged 4D gyrokinetic code which solves the Vlasov-Quasi-neutrality system. The code is based on a reduced model which averages out the cyclotron and the bounce motion of the trapped particles to reduce the dimensionality. In this work we developed and tested a new module for the code, allowing to track test particle trajectories in phase space. As a first result obtained with test particles, we achieved to separate the diffusive contribution to the radial particle flux in energy space, from the non-diffusive contributions. Both fluxes present an intense peak indicating resonant particles dominate transport. On short period of time the test particles undergo a small scale advection, but on longer times, they follow a random walk process. We then explored with greater accuracy the fluxes in energy space. Furthermore we compared the obtained fluxes with quasi-linear predictions and found a qualitative agreement, although there was a ~50% discrepancy in the peak magnitude.
