Continuum Damping of Toroidal Alfvén Eigenmodes in Finite-[beta] Tokamak Equilibria PDF Download

Author: Fulvio Zonca
Publisher:
ISBN:
Category :
Languages : en
Pages : 330

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Author:
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ISBN:
Category :
Languages : en
Pages : 77

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We have formulated a general theoretical approach for analyzing two-dimensional structures of high-n Toroidal Alfven Eigenmodes (TAE) in large aspect-ratio, finite-[beta] tokamaks. Here, n is the toroidal wave number and [beta] is the ratio between plasma and magnetic pressures. The present approach generalizes the standard ballooning-mode formalism and is capable of treating eigenmodes with extended global radial structures as well as finite coupling between discrete and continuous spectra. Employing the well-known (s, [alpha]) model equilibrium and assuming a linear equilibrium profile, we have applied the present approach and calculated the corresponding resonant continuum damping rate of TAE modes. Here, s and [alpha] denote, respectively, the strengths of magnetic shear and pressure gradients. In particular, it is found that there exists a critical [alpha]{sub c}(s), such that, as [alpha] 2![alpha]{sub c}, the continuum damping rate is significantly enhanced and, thus, could suppress the potential TAE instability.

Author: Liu Chen
Publisher:
ISBN:
Category : Magnetohydrodynamic waves
Languages : en
Pages : 76

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Author: George William Bowden
Publisher:
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Category :
Languages : en
Pages : 0

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A variety of Alfven wave phenomena are found in toroidal magnetically confined fusion plasmas. Shear Alfven eigenmodes may exist, which can be driven unstable by interaction with energetic particles. The linear stability of such modes depends on damping through several mechanisms. Continuum resonances cause damping of the modes, which occurs even in non-dissipative ideal magnetohydrodynamic (MHD ) theory given appropriate treatment of resulting poles. Additional damping of the modes occurs due to conversion to kinetic Alfven waves and finite parallel electric fields when kinetic extensions to MHD are considered. In this thesis, methods for calculating the damping of Alfven eigenmodes are developed, with particular focus on the continuum damping component. Damping of modes in complicated two- and three-dimensional magnetic geometries characteristic of tokamak and stellarator plasmas is considered.In this work, shear Alfven eigenmodes are analysed based on reduced MHD models. A background is provided, covering relevant theoretical aspects of plasma equilibrium, coordinate systems and linearised MHD waves. A coordinate independent reduced MHD wave equation is derived for Alfven eigenmodes in low beta tokamaks and stellarators. Coupled wave equations in terms of Fourier harmonics of the eigenmode are then derived for large aspect-ratio plasmas.Expressions for continuum damping are derived perturbatively from the coordinate independent and coupled harmonic wave equations. Application of the expressions using Galerkin and shooting methods is described. Damping computed in this manner is compared with values from an accepted method for the benchmark case of a TAE in a large aspect-ratio circular cross-section tokamak. The perturbative technique is shown to produce significant errors, even where continuum damping is small.A novel singular finite element method is developed to compute continuum damping. The Galerkin method adopted employs special basis functions reflecting the asymptotic form of the solution near continuum resonance poles. For particular eigenmodes, the unknown complex eigenvalue and pole location are computed iteratively. The procedure is verified by application to a TAE in a large aspect-ratio circular cross-section tokamak, where well converged and accurate complex eigenvalue and mode structure are obtained.Continuum damping can be computed numerically by solving the ideal MHD eigenvalue problem over a complex contour which circumvents continuum resonance poles according to the causality condition. This calculation is implemented in the ideal MHD eigenvalue code CKA , using analytic continuation of equilibrium quantities. The method is verified through application to a TAE in a tokamak, where the complex eigenvalue computed agrees closely with that found using the accepted resistive method, but converges faster with increasing radial mesh resolution. Continuum damping of shear Alfven eigenmodes is computed for three-dimensional configurations in torsatron, helias and heliac stellarators.Extensions to the ideal MHD wave equations allow non-ideal kinetic effects to be modelled. The damping of a TAE in a tokamak case through these effects is computed using different models for magnetic geometry and kinetic effects. Choice of the former strongly influences results, while choice of.

Author: C.Z. Cheng
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Category : Alpha rays
Languages : en
Pages : 52

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Category : Aeronautics
Languages : en
Pages : 892

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Category : Electronic journals
Languages : en
Pages : 532

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Author: Toshiki Tajima
Publisher: Springer
ISBN:
Category : Science
Languages : en
Pages : 360

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Annotation US and Japanese physicists from SSC accelerator, space, tokamak, and alternative fusion physics discuss the physical effects that are wrought on a plasma or a beam by the presence of high energy or energetic particles. The 23 papers cover generic processes involving high energy particles, ignited tokamaks, helical systems, field reversed configurations, and systems with large orbit particles. Reproduced from typescripts, some double spaced. Annotation c. by Book News, Inc., Portland, Or.

Author: Linjin Zheng
Publisher: BoD – Books on Demand
ISBN: 9535102117
Category : Science
Languages : en
Pages : 224

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To understand plasma physics intuitively one need to master the MHD behaviors. As sciences advance, gap between published textbooks and cutting-edge researches gradually develops. Connection from textbook knowledge to up-to-dated research results can often be tough. Review articles can help. This book contains eight topical review papers on MHD. For magnetically confined fusion one can find toroidal MHD theory for tokamaks, magnetic relaxation process in spheromaks, and the formation and stability of field-reversed configuration. In space plasma physics one can get solar spicules and X-ray jets physics, as well as general sub-fluid theory. For numerical methods one can find the implicit numerical methods for resistive MHD and the boundary control formalism. For low temperature plasma physics one can read theory for Newtonian and non-Newtonian fluids etc.

Author: Joseph Allan Snipes
Publisher:
ISBN:
Category :
Languages : en
Pages : 70

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Measured damping rates of stable Toroidal Alfvén Eigenmodes (TAEs) have been compared with damping rates calculated with the NOVA-K code for a number of experimental conditions on Alcator C-Mod in an attempt to validate the NOVA-K damping model. Very low amplitude ( ~ 10-7 at the wall) stable TAEs are excited in Ohmic plasmas with a pair of active MHD antennas inside C-Mod. By sweeping the excited frequency of the antennas through the expected TAE frequency, the frequency width of a mode resonance can be measured with poloidal field pick-up coils on outboard limiters to determine the damping rate of that mode. The calculated equilibrium and main plasma parameters at the time of the resonance are input into the NOVA-K code to then calculate the expected damping rate for the measured toroidal mode number and compare with the experimental value. The calculations include continuum damping, Landau damping on both the electrons and the background and impurity ions, and collisional damping on the electrons. Comparisons have been made for inner wall limited and lower single null diverted cases over a range of ITER-relevant moderate toroidal mode numbers ... Good agreement between the experiment and theoretical calculations can be obtained with reasonable q profiles for the plasma conditions in which perturbative NOVA models are applicable, but the results are found to depend very sensitively on the assumed q profile.