Internal Transport Barrier Triggered by Neoclassical Transport in W7-AS

Stroth, U.; Itoh, K.; Itoh, Sanae–I.; Hartfuß, H. J.; Laqua, H. P.; ECRH-Team,; Team, W As

doi:10.1103/physrevlett.86.5910

Cited by 100 publications

(111 citation statements)

References 19 publications

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“…7(c)) are moderately decreased. This finding has been interpreted [14,58] as the formation of an ITB triggered by the neoclassical transition in the ambipolar E r , and the reduction of "anomalous" transport driven by turbulence within the ITB has been attributed to the strong shear in the poloidal flow. (It should be noted, that the shear in the poloidal flow responsible for turbulence suppression is r(E r /r)' and not simply E r '.)…”

Section: E a Question Of Cause And Effectmentioning

confidence: 99%

Common Features of Core Electron-Root Confinement in Helical Devices

Yokoyama

Maaßberg

Beidler

et al. 2006

Fusion Science and Technology

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The characteristics of core electron-root confinement (CERC) in helical devices are illustrated using results from the four different experiments, CHS, LHD, TJ-II and W7-AS. Common features include strongly peaked electron temperature profiles and large positive radial electric fields, E r , in the core region for discharges with sufficient central electron cyclotron heating (ECH). Such observations are consistent with a transition to the "electron-root" solution of the ambipolarity condition for E r , a feature of neoclassical theory which is unique to non-axisymmetric configurations. The magnetic topology of the configuration plays a role in this transition and thresholds are found for the particle density and ECH power, in accordance with neoclassical expectations. Neoclassical theory alone cannot explain all observations, however, as CERC formation can also be influenced by ECH-driven convective fluxes of localized electrons and by the presence of magnetic islands in the core region. This is the first report describing collaborative activities within the framework of the International Stellarator Profile Data Base.2

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Section: E a Question Of Cause And Effectmentioning

confidence: 99%

Common Features of Core Electron-Root Confinement in Helical Devices

Yokoyama

Maaßberg

Beidler

et al. 2006

Fusion Science and Technology

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“…An internal transport barrier, that was found in CHS for the first time [16], becomes common in toroidal helical plasmas [17,18,19]. In a recent CHS experiment, the dual HIBPs succeeded to catch an exact moment of the back-transition.…”

Section: Introductionmentioning

confidence: 99%

Turbulence and transport characteristics of a barrier in a toroidal plasma

Fujisawa

Shimizu

Nakano

et al. 2006

Plasma Phys. Control. Fusion

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Abstract. Turbulence and zonal flow at a transport barrier are studied with twin heavy ion beam probes in a toroidal helical plasma. A wavelet analysis is used to extract turbulence properties, e. g., spectra of both density and potential fluctuations, coherence and phase between them, and the dispersion relation. Particle transport estimated from the fundamental characteristics is found to clearly rise with their intermittent activities after the barrier is broken down. The time-dependent analysis reveals that intermittency of turbulence is correlated with evolution of stationary zonal flow.

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“…Electron internal transport barriers (e-ITBs) are commonly observed in electron cyclotron heated (ECH) plasmas in stellarator devices such as CHS [1], W7-AS [2,3], LHD [4,5] and TJ-II [6,7]. e-ITBs are usually established in conditions of high ECH heating power density and are characterized by peaked electron temperature profiles with improved core electron heat confinement [1][2][3][4][5][6][7].…”

Section: Introductionmentioning

confidence: 99%

“…e-ITBs are usually established in conditions of high ECH heating power density and are characterized by peaked electron temperature profiles with improved core electron heat confinement [1][2][3][4][5][6][7]. In addition, a large radial electric field and shear in the inner region is measured [1][2][3][4]7] accompanied by a reduction of fluctuations [1]. Most of these experimental results support the hypothesis that the mechanism for barrier formation is linked to a bifurcation of the radial electric field E r while the subsequent reduction of turbulence is due to an enhancement of E r xB shearing flow.…”

Section: Introductionmentioning

confidence: 99%

Electron Internal Transport Barriers and Magnetic Topology in the Stellarator TJ-II

Estrada¹,

López‐Bruna²,

Alonso³

et al. 2006

Fusion Science and Technology

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In most helical systems electron Internal Transport Barriers (e-ITB) are observed in Electron Cyclotron Heated (ECH) plasmas with high heating power density. In the stellarator TJ-II, eITBs are easily achievable by positioning a low order rational surface close to the plasma core, because this increases the density range in which the e-ITB can form. Experiments with different low order rationals show a dependence of the threshold density and barrier quality on the order of the rational (3/2, 4/2, 5/3, ...). In addition, during the formation of e-ITB quasicoherent modes are frequently observed in the plasma core region. The mode can exist before or after the e-ITB phenomenon at the radial location of the transport barrier foot but vanishes as the barrier is fully developed. I. IntroductionElectron internal transport barriers (e-ITBs) are commonly observed in electron cyclotron heated (ECH) plasmas in stellarator devices such as CHS [1], W7-AS [2,3], LHD [4,5] and TJ-II [6,7]. e-ITBs are usually established in conditions of high ECH heating power density and are characterized by peaked electron temperature profiles with improved core electron heat confinement [1][2][3][4][5][6][7]. In addition, a large radial electric field and shear in the inner region is measured [1][2][3][4]7] accompanied by a reduction of fluctuations [1]. Most of these experimental results support the hypothesis that the mechanism for barrier formation is linked to a bifurcation of the radial electric field E r while the subsequent reduction of turbulence is due to an enhancement of E r xB shearing flow. A topical review on transport barriers has been published in [8] and a comparative study of transport barrier physics in different helical devices is reported in [9]. The specific characteristics of TJ-II, i.e. low magnetic shear and high flexibility with regard to the magnetic configuration allow us the control of low order rational surfaces within the rotational transform profile and, therefore, the study of how the magnetic topology affects e-ITB formation. Experimentally, a rational surface can be positioned in the plasma core region by selecting the appropriate magnetic configuration. In addition, the rotational transform profile can be modified dynamically during the discharge by inducing an ohmic current [10]. Experiments performed changing the magnetic configuration have shown that the presence of a low order rational surface (n=3/m=2) close to the plasma core triggers the e-ITB formation [7]. During the e-ITB formation, the electron temperature, measured by the ECE diagnostic

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Internal Transport Barrier Triggered by Neoclassical Transport in W7-AS

Cited by 100 publications

References 19 publications

Common Features of Core Electron-Root Confinement in Helical Devices

Common Features of Core Electron-Root Confinement in Helical Devices

Turbulence and transport characteristics of a barrier in a toroidal plasma

Electron Internal Transport Barriers and Magnetic Topology in the Stellarator TJ-II

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