Plasma pressure effect on Alfvén cascade eigenmodes

Breǐzman, B. N.; Pekker, M.; Sharapov, S. E.

doi:10.1063/1.2130692

Cited by 135 publications

(232 citation statements)

References 16 publications

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“…13]. This suggests the possibility of using the MiAE frequency scalings as a novel magnetic-island diagnostics in a fashion similar to other commonly used Alfvén spectroscopy techniques [24,25,26]. Unlike in the BAE case, the radial MiAE localization at the center of the island makes them difficult to detect by external measurements, and makes the use of internal fluctuation diagnostics, such as electron cyclotron emission and soft x rays, necessary.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

Shear Alfvén wave continuous spectrum within magnetic islands

Biancalani

Chen²,

Пегораро³

et al. 2010

Physics of Plasmas

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The radial structure of the continuous spectrum of shear Alfvén waves is calculated in this paper within the separatrix of a magnetic island. Geometrical effects due to the noncircularity of the flux surface's cross section are retained to all orders. On the other hand, we keep only curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum. Modes with different helicity from that of the magnetic island are considered. The main result is that, inside a magnetic island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and where a wide frequency gap is formed, analogous to the ellipticity induced Alfvén eigenmode gap in tokamaks. The presence of this gap is due to the strong eccentricity of the island cross section. The importance of the existence of such a gap is recognized in potentially hosting magnetic-island induced Alfvén eigenmodes (MiAE). Due to the frequency dependence of the shear Alfvén wave continuum on the magnetic-island size, the possibility of utilizing MiAE frequency scalings as a novel magnetic-island diagnostic is also discussed.

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Section: Conclusion and Discussionmentioning

confidence: 99%

Shear Alfvén wave continuous spectrum within magnetic islands

Biancalani

Chen²,

Пегораро³

et al. 2010

Physics of Plasmas

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“…RSAEs and other Alfvénic modes have been used for ''MHD spectroscopy''; that is, the inference of equilibrium parameters such as q min or q on axis (q 0 ) from frequency spectra [12,13]. The characteristic frequency sweep of the RSAEs has been well qualified in previous experimental [11,12], computational [14], and theoretical [15,16] studies. An approximate dispersion relation for the RSAEs can be found in Ref.…”

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confidence: 88%

Observation of Reversed Shear Alfvén Eigenmodes between Sawtooth Crashes in the Alcator C-Mod Tokamak

Porkoláb

Kramer

et al. 2009

Phys. Rev. Lett.

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Groups of frequency chirping modes observed between sawtooth crashes in the Alcator C-Mod tokamak are interpreted as reversed shear Alfvén eigenmodes near the q ¼ 1 surface. These modes indicate that a reversed shear q profile is generated during the relaxation phase of the sawtooth cycle. Two important parameters, q min and its radial position, are deduced from comparisons of measured density fluctuations with calculations from the ideal MHD code NOVA. These studies provide valuable constraints for further modeling of the sawtooth cycle. The sawtooth cycle has long been recognized as a robust feature of tokamak plasmas [1]. The cycle is composed of an abrupt crash in the central electron temperature and a relaxation, or reheat, phase (see Fig. 1, t ! 0:25 s). The sawtooth crash is a process whereby the magnetic field lines interior to the q ¼ 1 surface (where q ¼ rB =R 0 B is the magnetic safety factor, r is the minor radial coordinate, B is the toroidal magnetic field, R 0 is the tokamak major radius, and B is the poloidal magnetic field) reconnect to lines outside of the q ¼ 1 surface. In this process the magnetic field ''frozen-in'' laws are broken leading to rapid radial transport wherein the temperature, density, and current profiles are flattened [2][3][4]. Following the cessation of magnetic reconnection, the plasma relaxation phase is characterized by a nearly linear increase in the central electron temperature and an inward diffusion of parallel current formerly displaced by the reconnection. Understanding the evolution of the current profile and its relation to sawtooth phenomena is important, especially for future burning plasma experiments such as ITER, where an accurate model of the physics of the crash event and the subsequent relaxation process is crucial for developing predictive capability for sawtooth control. In this Letter we report observations of frequency chirping Alfvénic modes excited during the sawtooth cycle in Alcator C-Mod tokamak plasmas, herein identified as reversed shear Alfvén eigenmodes (RSAEs, also Alfvén cascades) located near the q ¼ 1 surface. RSAEs should be of great interest for future burning plasma experiments both for their use as a form of MHD spectroscopy and because Alfvénic modes may negatively impact the confinement of the energetic ions in the plasma core [5]. Importantly, the presence of RSAEs near the q ¼ 1 surface is interpreted as evidence of a hollow current profile during the sawtooth cycle.The phase contrast imaging (PCI) diagnostic [6], a type of internal reference beam interferometer, is an outstanding tool for the study of core localized Alfvénic activity. The output signal of the PCI system is a 1D image, decomposed into 32 elements of approximately 4.5 mm chord separation in the direction of the major radius, which is linearly proportional to the line integral of the electron density perturbations along the beam path. The reporting of q ¼ 1 RSAEs in Alcator C-Mod, and also in JET [7,8] (see also [9]), is due to the availability of diagnostics sensiti...

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“…The RSAE frequency is a strong function of the minimum value of q (q min ) in equilibria with reversed shear [11][12][13]. Reversed shear q profiles are known to be present during the current ramp phase as the Ohmic current diffuses toward the core and is transiently peaked off axis, a fact confirmed by the observation of RSAEs in the presence of energetic ions [13,14], MSE measurements [15,16], and numerical modeling [17].…”

Section: Introductionmentioning

confidence: 79%

“…where T e is the electron temperature, M i is the ion mass, and R 0 is the tokamak major radius [12]. The Alfvénic frequency is given by the expression…”

Section: A Diagnosticsmentioning

confidence: 99%

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Phase contrast imaging measurements of reversed shear Alfvén eigenmodes during sawteeth in Alcator C-Mod

et al. 2009

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Reversed shear Alfvén eigenmodes (RSAEs) have been observed with the phase contrast imaging diagnostic and Mirnov coils during the sawtooth cycle in Alcator C-mod 1 plasmas with minority ion cyclotron resonance heating. Both down-chirping RSAEs and up-chirping RSAEs have been observed during the sawtooth cycle. Experimental measurements of the spatial structure of the RSAEs are compared to theoretical models based on the code NOVA 2 , and used to derive constraints on the q profile. It is shown that the observed RSAEs can be understood by assuming a reversed shear q profile (up-chirping) or a q profile with a local maximum (down-chirping) with q ≈ 1.

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Plasma pressure effect on Alfvén cascade eigenmodes

Cited by 135 publications

References 16 publications

Shear Alfvén wave continuous spectrum within magnetic islands

Shear Alfvén wave continuous spectrum within magnetic islands

Observation of Reversed Shear Alfvén Eigenmodes between Sawtooth Crashes in the Alcator C-Mod Tokamak

Phase contrast imaging measurements of reversed shear Alfvén eigenmodes during sawteeth in Alcator C-Mod

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