Transitions to improved core electron heat confinement triggered by low order rational magnetic surfaces in the stellarator TJ-II

Estrada, T.; Medina, F.; López‐Bruna, D.; Ascasíbar, E.; Balbín, Rosa; Cappa, Á.; Castejón, F.; Eguilior, Sonsoles; Fernández, Á.; Guasp, J.; Hidalgo, C.; Petrov, S. Ya.

doi:10.1088/0029-5515/47/4/009

Cited by 35 publications

(36 citation statements)

References 23 publications

(40 reference statements)

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“…The latter may contribute to the formation of an internal transport barrier [11,12]. External resonant magnetic perturbations (RMPs) can generate not only stochastic magnetic fields, but also coherent magnetic islands [13].…”

Section: Introductionmentioning

confidence: 99%

Plasma flows and fluctuations with magnetic islands in the edge plasmas of J-TEXT tokamak

et al. 2015

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Abstract:The first comprehensive measurements of plasma flows and fluctuations nearby static magnetic islands driven by resonant magnetic perturbations (RMPs) are presented.These experiments were performed using multiple Langmuir probe arrays in the edge plasmas of the J-TEXT tokamak. The effects of controlled variations of the island size and location are explored. This study aims to understand the interaction between turbulence and magnetic islands, and to elucidate magnetic island effects on edge turbulence and flow intensity profiles, edge electric fields, and thus confinement regime transitions. Turbulence and low frequency flows (LFFs) all drop inside the magnetic island, but increase at its boundary, as island width increases. The geodesic acoustic mode (GAM) is damped in most of the edge area with magnetic islands. The sign of the radial electric field changes from negative to positive within 2 the islands. The gradient of turbulent stresses vanishes at island center, and becomes steeper at the boundary of the islands. The particle transport induced by the turbulence is reduced inside the magnetic islands. The magnetic island effects on flows and turbulence can lead to an increase in LFFs and enhance Reynolds stresses near the last closed flux surface (LCFS). A stronger radial electric field layer can be formed near the LCFS when magnetic islands are present. The results suggest that magnetic islands can be used as a tool to enhance edge turbulence and flows, edge electric fields, and thus to trigger confinement regime transitions.

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Section: Introductionmentioning

confidence: 99%

Plasma flows and fluctuations with magnetic islands in the edge plasmas of J-TEXT tokamak

et al. 2015

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“…In the large helical device (LHD) [18][19][20][21][22] and the TJ-II, 23 spontaneous shrinkage of RMP-driven magnetic islands has been observed, known as the self-healing. It has been pointed out that the perturbed bootstrap current, the polarization current, and the curvature-driven current are hopeless to explain the self-healing mechanism in low-b regime, 18,20,21 where b is a ratio of the total plasma pressure and the magnetic pressure.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear stability of magnetic islands in a rotating helical plasma

et al. 2012

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Linear and nonlinear wave propagation in weakly relativistic quantum plasmas Phys. Plasmas 20, 012114 (2013) Time-domain simulation of nonlinear radiofrequency phenomena Phys. Plasmas 20, 012116 (2013) Small amplitude nonlinear electron acoustic solitary waves in weakly magnetized plasma Phys. Plasmas 20, 012113 (2013) Dust-acoustic shock formation in dusty plasmas with non-thermal ions Phys. Plasmas 20, 013704 (2013) Additional information on Phys. Plasmas Coexistence of the forced magnetic reconnection by a resonant magnetic perturbation (RMP) and the curvature-driven tearing mode is investigated in a helical (stellarator) plasma rotated by helical trapped particle-induced neoclassical flows. A set of Rutherford-type equations of rotating magnetic islands and a poloidal flow evolution equation is revisited. Using the model, analytical expressions of criteria of spontaneous shrinkage (self-healing) of magnetic islands and sudden growth of locked magnetic islands (penetration of RMP) are obtained, where nonlinear saturation states of islands show bifurcation structures and hysteresis characteristics. Considering radial profile of poloidal flows across magnetic islands, it is found that the self-healing is driven by neoclassical viscosity even in the absence of micro-turbulence-induced anomalous viscosity.

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“…Bifurcation of equilibria between with and without magnetic islands is observed in helical devices such as the LHD [1, 2] and the TJ-II [3], where the tearing mode is linearly stable. In particular, spontaneous annihilation of magnetic islands called 'self-healing' triggers the improvement of plasma confinements.…”

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confidence: 99%

Forced Magnetic Reconnection in Helical Plasmas

Nishimura

Narushima

Toda

et al. 2010

Plasma and Fusion Research

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Magnetic islands excited by resonant magnetic perturbations (RMPs) in helical plasmas are investigated. A Rutherford-type equation is coupled with the time evolution equation of the radial electric field associated with the neoclassical particle diffusion due to helically rippled magnetic fields. Using the model, bifurcation between the excitation and the annihilation of non-rotating magnetic islands are newly observed, depending on the magnitude of RMPs and the anomalous plasma viscosity. It is found that the transition between these states is triggered by the change in the radial electric field profile in the vicinity of magnetic island. Keywords: magnetic island, radial electric field, plasma rotation, neoclassical particle diffusion DOI: 10.1585/pfr.5.040Forced magnetic reconnection due to resonant magnetic perturbations (RMPs) is of great interest in magnetic fusion plasmas. Bifurcation of equilibria between with and without magnetic islands is observed in helical devices such as the LHD [1, 2] and the TJ-II [3], where the tearing mode is linearly stable. In particular, spontaneous annihilation of magnetic islands called 'self-healing' triggers the improvement of plasma confinements. Magnetic islands and stochastic layers in helical plasmas have been investigated in the context of the three-dimensional resistive magnetohydrodynamics (MHD) equilibrium [4], and mechanism of the self-healing is under investigation. Such bifurcation is also observed in tokamaks, and theoretical works reveal that poloidal and toroidal rotations of plasmas play key roles, screening the penetration of RMPs [5]. In helical plasmas, the poloidal rotation is driven by E × B and diamagnetic drifts, where radial electric fields are produced by the neoclassical particle diffusion associated with helically rippled magnetic fields [6]. Therefore, the poloidal rotation might be associated with the self-healing mechanism. In order to test this hypothesis, theoretical models including finite Lamor radius (FLR) effects, such as the neoclassical particle diffusion, are necessary.In this paper, we introduce a simple theoretical model describing the forced magnetic reconnection in helical plasmas, and the bifurcation mechanism associated with the poloidal rotation is examined.The derivation of the model equations is outlined below. We start from two-fluids equations including FLR effects based on the drift ordering [7]. Perpendicular electric current is composed of the polarization current, the diamagnetic current and currents due to anisotropic presauthor's e-mail: nishimura.seiya@lhd.nifs.ac.jp sure tensors. Neoclassical particle fluxes are perturbatively included by substituting gyrophase-averaged distribution functions given by linearized drift-kinetic equations into moment formulae of anisotropic pressure tensors. We adopt a simplified version of the radial neoclassical particle fluxes given in Ref. [8]. For parallel electric currents, we focus on the regime where the bootstrap current is less important than the inductive current. In fact, ...

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Transitions to improved core electron heat confinement triggered by low order rational magnetic surfaces in the stellarator TJ-II

Cited by 35 publications

References 23 publications

Plasma flows and fluctuations with magnetic islands in the edge plasmas of J-TEXT tokamak

Plasma flows and fluctuations with magnetic islands in the edge plasmas of J-TEXT tokamak

Nonlinear stability of magnetic islands in a rotating helical plasma

Forced Magnetic Reconnection in Helical Plasmas

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