Alfven wave coupling experiments of the TCA Tokamak

De.Chambrier, A.; Cheetham, A.D.; Heym, A.; Hofmann, F.; Joye, B.; Keller, R.; Lietti, A.; Lister, J.B.; Pochelon, A.

doi:10.1088/0032-1028/24/8/003

Cited by 46 publications

(18 citation statements)

References 7 publications

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“…The minima in A 2 are typically very shallow in W7-AS, and therefore the formation of radially extended mode structures may be expected. Extensive theoretical [21][22][23][24][25][26][27] and experimental [27][28][29][30] work has been made, particularly with regard to the possibility of using GAE resonances for heating and current drive in tokamaks.…”

Section: Fast Particle Driven Global Alfvé N Eigenmodesmentioning

confidence: 99%

Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

et al. 2001

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Magnetohydrodynamic (MHD) instabilities in the Wendelstein 7-AS stellarator (W7-AS) [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] are characterized experimentally in various plasma parameter regimes and heating scenarios. The observations are compared with theoretical predictions for particular cases. In the high-β range (〈β〉⩽2%) no clear evidence of a stability β-limit could be found yet. In the lower β regime fast particle driven global Alfvén modes are the most important instabilities during neutral beam injection (NBI). Besides of coherent modes with almost no effect on the plasma performance additional Alfvén modes appear at higher frequencies up to 400 kHz, which show nonlinear phenomena-like bursting, frequency chirping, and MHD induced energy and fast particle losses. The activity of edge localized modes (ELMs) is investigated in NBI heated discharges. The issue of current driven instabilities and their potential stabilization by a stellarator field has been investigated with regard to the design of compact hybrid stellarator systems.

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Section: Fast Particle Driven Global Alfvé N Eigenmodesmentioning

confidence: 99%

Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

et al. 2001

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“…Because of their relatively weak damping, the GAE can be excited to high amplitudes with an external antenna and have been previously observed as resonances in the plasma loading resistance. 3 They make up the lowfrequency part of the stable discrete spectrum of magnetohydrodynamics. 4,5 They owe their existence entirely to plasma inhomogeneities, i.e., to the coupling between the shear and compressional modes brought about by gradients in the equilibrium current and density.…”

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confidence: 99%

Direct Observation of the Structure of Global Alfvén Eigenmodes in a Tokamak Plasma

et al. 1984

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We present the first direct observation of the structure of a driven global Alfven eigenmode in a tokamak plasma using C0 2 laser interferometry.PACS numbers: 52.40.Db, 52.50.Gj The properties of Alfven waves in hot, magnetically confined plasmas are quite unlike those in homogeneous media. Despite their significance for the interpretation of both laboratory and astrophysical phenomena, experimental studies of these waves have been limited principally to the determination of the cavity eigenmode frequencies or parallel phase velocities. With the development of long confinement times in fusion experiments, laser interferometry to measure density fluctuations, and a theoretical understanding of the density fluctuations associated with Alfven waves, it has now become possible to investigate their spatial structure.We report the first such investigation, 1,2 which identifies and determines the structure of global Alfven eigenmodes (GAE). These modes are particular manifestations of the effects of magnetic confinement on the Alfven waves. Because of their relatively weak damping, the GAE can be excited to high amplitudes with an external antenna and have been previously observed as resonances in the plasma loading resistance. 3 They make up the lowfrequency part of the stable discrete spectrum of magnetohydrodynamics. 4,5 They owe their existence entirely to plasma inhomogeneities, i.e., to the coupling between the shear and compressional modes brought about by gradients in the equilibrium current and density. Rediscovered as broadened resonances in calculations based on kinetic theory, 6 the GAE have been further studied, both analytically and numerically, as candidates for the heating of fusion plasmas. 7 " 11 To model the GAE in a tokamak, we assume a cylindrical plasma of length 2TTR. The characteristic frequencies of the GAE are denoted by co /m , where / and m are the axial (toroidal) and azimuthal (poloidal) mode numbers, respectively. The plasma has axial magnetic field B 0 z and current density Jo(r), which determines the poloidal field B 0 (r), and hence the safety factor q(r) = rBjRB 9 . The wave number parallel to the field is k\\{r) = (-/+ m/q)/R, and the shear Alfven frequency is defined by co A 0) = k\\v K , where v A 0) = 2V {ponm Q ft) xl1 , n = n(r) is the plasma density, and w eff is an effective mass which depends on the impurity concentration. To include finite-ion-gyrofrequency corrections, we use the same mass, taking co ci = eBo/m eff .The characteristic frequencies of the GAE lie below the threshold of the spatial Alfven-cyclotron resonance or the Alfven continuum, defined by co 2 (l -a) 2 /(Oct) = (*)\. n That is, for these modes, / and m have opposite signs so that k\\ is nonzero and \ < 0 everywhere in the plasma. For each l,m there is an infinite sequence of eigenmodes, which may be denoted by a radial mode number. We omit this label here and consider only the lowest frequency or fundamental mode. The other modes tend to merge into the continuum as a resu...

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“…Also known as discrete Alfvén waves (DAWs), they were discovered almost simultaneously in experimental measurements [1] and numerical computations [2]. Dismissed as a potential heating scheme for fusion plasmas [2], AEs have in recent times received considerable renewed theoretical and experimental attention in the light of questions raised as to their stability in the presence of fast ions [3][4][5], such as fusion born alpha particles and ions from neutral beam injection (NBI), or ion cyclotron resonance heating (ICRH).…”

Section: Introductionmentioning

confidence: 99%

A complete asymptotic expansion of the spectral function for second-order elliptic operators inRⁿ

Popov¹,

Shubin²

1983

Russ. Math. Surv.

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The low shear in W7-AS magnetic configurations leads to global Alfvén eigenmodes (GAEs) of both helicities. Thus, low frequency GAEs with the same helicity as toroidal Alfvén eigenmodes (TAEs) and elliptical Alfvén eigenmodes (EAEs) in tokamaks exist in W7-AS; GAEs that are usually forbidden in tokamaks. Observations of diagnostic signals reveal a rich spectrum of neutral beam injection (NBI) driven GAEs in W7-AS in the frequency range 0 ≤ fGAE ≤ 500 kHz. These GAEs are excited via sideband resonances with NBI ions at low plasma densities and via fundamental resonances at higher plasma densities. A transition in the mode of excitement occurs when the increasing plasma density brings the central local Alfvén velocity down below the maximum tangential injection velocity of the NBI ions.

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Alfven wave coupling experiments of the TCA Tokamak

Cited by 46 publications

References 7 publications

Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

Direct Observation of the Structure of Global Alfvén Eigenmodes in a Tokamak Plasma

A complete asymptotic expansion of the spectral function for second-order elliptic operators inRⁿ

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Alfven wave coupling experiments of the TCA Tokamak

Cited by 46 publications

References 7 publications

Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

Direct Observation of the Structure of Global Alfvén Eigenmodes in a Tokamak Plasma

A complete asymptotic expansion of the spectral function for second-order elliptic operators inRn

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A complete asymptotic expansion of the spectral function for second-order elliptic operators inRⁿ