Influence of Low‐Order Rational Surfaces on the Radial Electric Field of TJ‐II ECH Plasmas

Bondarenko, O.; Estrada, T.; Jiménez‐Gómez, R.; López‐Bruna, D.; Happel, T.; Romero, Jesús; Löpez‐Fraguas, A.; Ascasíbar, E.; Blanco, E.

doi:10.1002/ctpp.200900026

Cited by 18 publications

(16 citation statements)

References 17 publications

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“…Under these conditions a reduction of the effective power threshold due to the rational surface could explain the pronounced iota-dependence found in this magnetic configuration scan, e.g., as the magnetic resonance approaches the edge plasma region (ρ ≈ 0.8), the corresponding local modification of the radial electric field can enhance the E r × B sheared flow so the transition takes place at a power level that is below the threshold without resonance. Experiments performed in low density ECH plasmas point to the modification of the ambipolar radial electric field due to the magnetic resonances as they approach either the plasma edge region [20] or the plasma core [21].…”

Section: Magnetic Configuration Scanmentioning

confidence: 99%

L‐H Transition Experiments in TJ‐II

Estrada¹,

Ascasíbar

Happel

et al. 2010

Contrib. Plasma Phys.

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In the TJ-II stellarator, L-to H-mode transitions are achieved under NBI heating conditions when operating with lithium coated walls. Several magnetic configurations have been explored in order to study the influence of the rotational transform in the transition. The shear of the negative radial electric field, E r , increases at the L-H transition by an amount that depends on the magnetic configuration and heating power. Magnetic configurations with and without a low order rational surface close to the plasma edge show differences that may be interpreted in terms of local changes in the radial electric field induced by the rational surface that could facilitate the L-H transition. Fluctuation measurements show a reduction in the turbulence level that is strongest at the position of maximum Er shear. High temporal and spatial resolution measurements indicate that turbulence reduction precedes the increase in the mean sheared flow, but is simultaneous with the increase in the low frequency oscillating sheared flow. These observations may be interpreted in terms of turbulence suppression by oscillating flows, the so-called zonal flows.

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Section: Magnetic Configuration Scanmentioning

confidence: 99%

L‐H Transition Experiments in TJ‐II

Estrada¹,

Ascasíbar

Happel

et al. 2010

Contrib. Plasma Phys.

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“…It could be connected to small changes in the magnetic configuration: the toroidal current I t ∼ kA changes during the duration of the discharge, so does the rotational transform, and low-order rationals may enter or exit the plasma. Rationals are known to modify the local electric field [58,59], but the event detected by bolometry is global. Rationals aside, the effect of a change of rotational transform in the radial electric field is expected to be negligible in ion root [60], as compared with the changes measured in figure 8.…”

Section: Tj-iimentioning

confidence: 99%

Section: Tj-iimentioning

confidence: 99%

Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices

Velasco¹,

Calvo²,

Satake³

et al. 2016

Nucl. Fusion

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Achieving impurity and helium ash control is a crucial issue in the path towards fusion-grade magnetic confinement devices, and this is particularly the case of helical reactors, whose low-collisionality ion-root operation scenarios usually display a negative radial electric field which is expected to cause inwards impurity pinch. In this work we discuss, based on experimental measurements and standard predictions of neoclassical theory, how plasmas of very low ion collisionality, similar to those observed in the impurity hole of the Large Helical Device [1-3], can be an exception to this general rule, and how a negative radial electric field can coexist with an outward impurity flux. This interpretation is supported by comparison with documented discharges available in the International Stellarator-Heliotron Profile Database, and it can be extrapolated to show that achievement of high ion temperature in the core of helical devices is not fundamentally incompatible with low core impurity content.

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“…The fact that the rotation approaches zero in the case of figure 3a is incidental. From several measurements of this kind we can conclude that there is a radial electric field in the region occupied by the resonance contributing always towards positive; in other words, if the ambient -in the absence of resonanceselectric field is negative, it tends to zero or positive in the presence of the resonance; if the ambient field is positive, it strengthens [14]. Finally, it is noteworthy that the density fluctuation level decreases significantly while the change in rotation velocity is detected.…”

Section: Rotational Transform Scansmentioning

confidence: 94%

Magnetic Resonances in ECR‐Heated Plasmas of the TJ‐II Heliac

López‐Bruna

Romero

Löpez‐Fraguas

et al. 2010

Contrib. Plasma Phys.

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Low order rational values of the rotational transform -magnetic resonances, for brevity-can be present inside ECR-heated plasmas of the TJ-II Heliac, in low magnetic shear conditions, without causing damage to confinement. Moreover, in agreement with previous experience in the TJ-II, the resonances seem to benefit confinement and are associated to changes in the radial electric field, at least in the density gradient region. These results encourage considering magnetic resonances as possible external actuators on confinement in stellarator/heliotron devices.

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Influence of Low‐Order Rational Surfaces on the Radial Electric Field of TJ‐II ECH Plasmas

Cited by 18 publications

References 17 publications

L‐H Transition Experiments in TJ‐II

L‐H Transition Experiments in TJ‐II

Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices

Magnetic Resonances in ECR‐Heated Plasmas of the TJ‐II Heliac

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