Observation of MHD structures in JET temperature profiles

Nave, M. F. F.; Edwards, A. W.; Hirsch, K.; Hugon, M.; Jacchia, A.; Lazzaro, E.; Salzmann, H.; Smeulders, P.

doi:10.1088/0029-5515/32/5/i09

Cited by 53 publications

(34 citation statements)

References 26 publications

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“…In the case of the (m,n)=(2,1) mode, however, the situation is more ambiguous. Flat spots in the T e profile are consistently observed at high T e (T e ≥ 500 eV) on JET [3,5,7] and TFTR [8]. In [9] observations of flat spots in the density profile in TEXT are reported.…”

Section: Introductionmentioning

confidence: 81%

“…In general, the fact that sometimes no flattening is observed can be simply explained by the (relative) ineffectiveness of the transport along the field lines. The fact that in some experiments [3,5,7,8,9] flat spots have been observed in (m,n) = (2,1) islands, whereas in others [4,6,10] only reduced gradients or no flat spots at all have been observed, may serve as an indication that α in general is close to its critical value, so that the occurrence of flat spots depends rather sensitively on specific discharge conditions. This could also explain why, in our case, in the higher T e region the temperature contours follow the flux contours quite well, while in the lower T e part of the island the two are completely uncorrelated.…”

Section: Modelling Transport Across Anmentioning

confidence: 99%

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Gradients of electron temperature and density across m = 2 magnetic islands in RTP

Lammeren¹,

Cardozo²,

Schüller³

et al. 1993

Nucl. Fusion

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ABSTRACT. Simultaneous measurements of oscillations of the electron temperature, the electron density and the poloidal magnetic field in the Rijnhuizen Tokamak RTP are combined to obtain insight into the structure of MHD mode perturbations. Diagnostics used are a 20 channel heterodyne ECE radiometer, a 19 channel FIR interferometer, and a set of magnetic pick-up coils. A m/n = 2/1 island preceding a disruption is analysed in detail. Whereas the density and temperature are significantly lower in the O-point than in the X-point, no local minimum or even flattening of the profiles could be demonstrated. The gradients across the island may be understood in terms of a very long connection length inside the island, in combination with the relatively short mean free path along the field lines due to the high collisionality in the outer region of the plasma. The transport inside the island and the poloidal distribution of the radial heat flux over X-and O-point are discussed.

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Section: Introductionmentioning

confidence: 81%

Section: Modelling Transport Across Anmentioning

confidence: 99%

Gradients of electron temperature and density across m = 2 magnetic islands in RTP

Lammeren¹,

Cardozo²,

Schüller³

et al. 1993

Nucl. Fusion

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“…Our interest is toward what happens to these vacuum magnetic configurations when plasma is produced. Although the exact magnetic configuration is difficult to measure directly with the presently available diagnostics, we may infer the state of a magnetic configuration from the fine-spatial profile of electron temperature (T e ), which is supposed to be constant along a magnetic field line, as was demonstrated in tokamaks [13,14]: If a T e profile has a flat region or a bump near a rational surface, this probably implies an island, though the possibility of a very local heating or a local transport anomaly cannot be ruled out. The Thomson scattering system [15] installed on LHD was designed to repetitively measure T e and electron density (n e ) at 200 positions along the major radius on the Z 0 plane at the horizontally elongated section.…”

mentioning

confidence: 99%

Observation of the “Self-Healing” of an Error Field Island in the Large Helical Device

Narihara¹,

Watanabe²,

Yamada³

et al. 2001

Phys. Rev. Lett.

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It was observed that the vacuum magnetic island produced by an external error magnetic field in the large helical device shrank in the presence of plasma. This was evidenced by the disappearance of flat regions in the electron temperature profile obtained by Thomson scattering. This island behavior depended on the magnetic configuration in which the plasmas were produced. DOI: 10.1103/PhysRevLett.87.135002 PACS numbers: 52.55.HcExistence of a set of nested magnetic surfaces (magnetic configuration) is a prerequisite for almost all toroidal magnetic confinement devices [1]. In the tokamak, a magnetic configuration is generated by plasma current plus external coil currents, and hence the concept of a vacuum magnetic configuration does not exist. On the other hand, the stellarator, by its definition, possesses a vacuum magnetic configuration that is able to confine plasma just as Nature possesses a vacuum space-time that allows the existence of matter. This fact makes it easier in the stellarator than in the tokamak to study the interactions between the magnetic configuration and plasma such as the formation of topological defects and their reactions on the plasma behavior. The vacuum magnetic configuration of a stellarator is inevitably accompanied by topological defects such as magnetic islands and stochastic regions as a result of a weak violation of a helical symmetry. If these defects are large in size or numerous, they will deteriorate the plasma confinement considerably. This was a central issue in stellarator development. However, nowadays, we can make these topological defects small enough not to degrade the plasma confinement over a wide region by optimizing the coil-winding law [2]. Although the vacuum magnetic configuration problem was thus solved, the question whether a fairly good magnetic configuration is preserved in the presence of plasma has not yet been settled. Cary and Kotschenreuther [3] showed that sharply peaked currents near the island enhance or limit the island size, depending on whether the average curvature is bad or good. They also showed that in the hill region the islands overlap for arbitrary small beta b (kinetic pressure/magnetic pressure). Hegna and Bhattacharjee [4] generalized the above statement for higher b plasma with the result that the well/hill criterion should be replaced by the resistive interchange stability criterion. Hayashi et al. [5] computationally found that the vacuum magnetic surfaces of an L 2͞M 10 heliotron/torsatron configuration are, indeed, broken at high b, giving a more stringent limit on b than that imposed by equilibrium. Hayashi et al. [6] also discovered that as b increases the islands present in the vacuum heliac configuration diminish ("self-heal") and then reappear with the flipped phase at a higher b. Bhattacharjee et al. [7] formulated a theory to explain the above "self-healing" phenomenon. Hegna [8] further generalized the above theories to incorporate the effect of a bootstrap current. Since these theoretical and numerical predictions hav...

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“…The coexistence of tearing modes with different helicity further complicated not only the sawtooth, but also the disruption instability [20][21][22][23][24]. The mechanisms of disruptions have not been well understood from the view point of MHD instability up to the present.…”

mentioning

confidence: 99%

Understanding complex magnetohydrodynamic activities associated with a relaxation in the HT-7 tokamak

Igochine

et al. 2011

Plasma Phys. Control. Fusion

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A new relaxation instability with complex magnetohydrodynamics (MHD) activities has been found in the HT-7 tokamak operational region, which manifests itself in the bursts of hydrogen alpha-ray radiations, electron cyclotron emission (ECE), and soft x-ray (SX) radiations on outer channels, as well as the complex MHD perturbations, but without hard disruptions. The MHD modes were indentified as m/n=2/1, m/n=3/2, and m/n=5/3 just before the relaxation (m and n are poloidal and toroidal mode number, respectively). The Poincaré mapping, according to the measured perturbations of the MHD modes at edge, was used to show the topology of magnetic field lines. It was found that a stochastic annular belt was resulted just before the relaxation due to the m/n=5/3 island 2 overlapping with m/n=2/1 and m/n=3/2 islands. That qualitatively coincided with the results of SX tomography.

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Observation of MHD structures in JET temperature profiles

Cited by 53 publications

References 26 publications

Gradients of electron temperature and density across m = 2 magnetic islands in RTP

Gradients of electron temperature and density across m = 2 magnetic islands in RTP

Observation of the “Self-Healing” of an Error Field Island in the Large Helical Device

Understanding complex magnetohydrodynamic activities associated with a relaxation in the HT-7 tokamak

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