Author: Zhisong Qu
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Category :
Languages : en
Pages : 0
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External heating methods such as neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) generate a large amount of fast ions in tokamak plasmas. The widely implemented MHD single fluid theory with isotropic pressure is no longer sufficient to capture the physics of such plasmas. Despite the shortcoming of a fluid theory, such as the fluid closure problem and the lack of wave-particle interactions, the use of a fluid description in a tokamak with external heated fast ions is possible and has proved fruitful due to its simple and intuitive nature, as shown in this thesis. Due the presence of the fast ions, the total plasma pressure becomes anisotropic. In other words, the pressure parallel to the magnetic field differs from its perpendicular counterpart. We have upgraded the fast ion driven instability tool chain HELENA-MISHKA-HAGIS to new versions with pressure anisotropy, taking the simplification that the whole plasma (electrons, fast and thermal ions) is a bi-Maxwellian fluid. Based on this new tool chain and analytical analysis, we have identified the impact of pressure anisotropy induced by externally heated fast ions on the plasma equilibrium, waves and instabilities. It has been found that if an isotropic model is used to describe an anisotropic plasma, a range of problems will emerge depending on the inverse aspect ratio and the magnitude of anisotropy. These problems include the inconsistency of the poloidal (diamagnetic) current, the constant pressure surface shifting away from the flux surfaces, and finally a distortion of the current and q profile. Two MAST experimental discharges are analysed, while in one of them, #29221@190ms, all three problems are presented, confirming the prediction. The equilibrium reconstructions for this discharge with/without anisotropy give different q profiles. This difference in the q profile leads to different continua, different n=1 TAE mode structures, and finally, different growth rates and saturation levels. The tool chain has also been used to carry on other physics studies such as an investigation of the dependency of the continuous spectra on different fluid closures and level of anisotropy. In addition to the waves that are supported by the thermal plasma, and modified and driven unstable by the fast ions, there are a family of waves, the energetic particle modes (EPMs), whose existence and property are determined by the fast ions, such as the energetic geodesic acoustic modes (EGAMs). The EGAMs are m=n=0 bursting and chirping modes first observed in DIII-D counter beam experiments. By considering the fast ions as a fluid with a collective flow along the field lines, we have reached a dispersion relationship that gives an unstable branch at half of the thermal GAM frequency. We have also found that when the beam is cold, there is a good agreement between our fluid theory and the existing kinetic theories. However, since the fluid theory does not capture the physics of inverse Landau damping, the source of the instability must be reactive, in contrast to the previous understandings. Furthermore, a smooth transition between the reactive EGAMs and the wave-particle interaction driven EGAMs is found when the beam temperature gradually increases, resembling the transition between the two-stream instability and the bump-on-tail instability in a beam-plasma system. This local fluid model is then extended to a global one to capture the physics of EGAM radial mode structure in the regime where fast ion drift orbit width is smaller than the mode width. The dependency of the mode structure on the equilibrium q profiles and the beam injection direction is investigated. By demonstrating the above two applications of the fluid theory and the corresponding physics discoveries, we have proved the usefulness of a fluid treatment in tokamak plasmas with external heating, serving to understanding some of the basic fast ions physics and acting as a powerful and indispensable complement to its kinetic counterpart.
Author: Camille Gillot
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Languages : en
Pages : 0
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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.
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Category : Aeronautics
Languages : en
Pages : 804
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Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
Author: Hugo Bufferand
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Languages : en
Pages : 180
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In the scope of designing future nuclear fusion reactors, a clear understanding of the plasma-wall interaction is mandatory. Indeed, a predictive estimation of heat flux impacting the surface and the subsequent emission of impurities from the wall is necessary to ensure material integrity and energy confinement performances. In that perspective, the fluid code SolEdge2D has been developed to simulate plasma transport in the tokamak edge plasma. The plasma-wall interaction is modeled using an innovative penalization technique. This method enables in particular to take complex plasma facing components geometry into account. In parallel to this numerical effort, a theoretical work has been achieved to find appropriate corrections to fluid closures when collisionality drops. The study of stochastic 1D models has been realized in collaboration with physicists from the CSDC group in Florence. A generalized Fourier law taking long range spatio-temporal correlations has been found to properly account for ballistic transport in the low collisional regime. This formulation is expected to be used to model parallel heat flux or turbulent cross-field transport in tokamak plasmas.
Author: Diethelm Duechs
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Category :
Languages : en
Pages : 40
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The Tokamak system is a toroidal electromagnetic field plasma configuration in which the magnetic field ratio B sub z/B sub theta is large. This toroidal configuration, which is one of the simpler magnetic confinement geometries, has led to relatively high plasma temperatures, densities, and containment times. The growing amount of experimental data, which needs to be explained, reveals the need for complicated theoretical plasma models similar to those which have been applied to pinch plasmas over the past several years. It does not seem possible to explain the experimental data by using the present two-fluid model applying the usual (classical) transport coefficients. Two major model expansions are obvious: (a) increase the number of fluids in the model, and (b) take into account as many spatial dimensions as possible. A fluid model that includes neutrals, electrons, and ions with arbitrary charge Z is derived. Cylindrical symmetry is imposed, although the transport coefficients include corrections for toroidal geometry. Assumptions are discussed under which this model can be applied to describe a Tokamak plasma consisting of neutral hydrogen, protons, electrons, and nine ionization stages of oxygen impurities. (Author).
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Category : Controlled fusion
Languages : en
Pages : 170
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Author: National Research Council
Publisher: National Academies Press
ISBN: 0309035481
Category : Science
Languages : en
Pages : 337
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Author: United States. Superintendent of Documents
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Category : Government publications
Languages : en
Pages : 1220
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Author:
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Category : Power resources
Languages : en
Pages : 508
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Author:
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Category : Government publications
Languages : en
Pages : 1228
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