Enhanced heat confinement in the flexible heliac TJ-II

Castejón, F.; Tribaldos, V.; Garcı́a-Cortés, I.; Luna, E. de la; Herranz, J.; Pastor, I.; Estrada, T.; Team, Tj‐Ii

doi:10.1088/0029-5515/42/3/307

Cited by 63 publications

(74 citation statements)

References 28 publications

(26 reference statements)

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“…The ECH power threshold is found to increase with n e [26][27][28]. Consequently, the n e threshold for fixed P ECH is also demonstrated at LHD [28], TJ-II [29] and W7-AS [30].…”

Section: B=152t) Where the Normalized Scale Length Of The T E Gradimentioning

confidence: 70%

Core electron-root confinement (CERC) in helical plasmas

et al. 2007

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Abstract. The improvement of core electron heat confinement has been realized in a wide range of helical devices such as CHS, LHD, TJ-II and W7-AS. Strongly peaked electron temperature profiles and large positive radial electric field, E r , in the core region are common features for this improved confinement. Such observations are consisitent with a transition to the "electron-root" solution of the ambipolarity condition for E r in the context of the neoclassical transport, which is unique to non-axisymmetric configurations. Based on this background, this improved confinement has been collectively dubbed "core electron-root confinement" (CERC). The thresholds for CERC establishment are found for the collisionality and electron cyclotron heating (ECH) power.The magnetic configuration properties (e.g., effective ripple and magnetic islands/rational surfaces) play important roles for CERC establishment.

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“…The ECH power threshold is found to increase with n e [26][27][28]. Consequently, the n e threshold for fixed P ECH is also demonstrated at LHD [28], TJ-II [29] and W7-AS [30].…”

Section: B=152t) Where the Normalized Scale Length Of The T E Gradimentioning

confidence: 70%

Core electron-root confinement (CERC) in helical plasmas

et al. 2007

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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].…”

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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“…In either case, the heating power is coupled to the electrons (in NBI because of the high beam energy E beam = 35 keV) The ion temperature is always limited to about T i ≈ 200 eV; at low densities because of the bad electron-ion coupling [31] and at higher densities because of the large ion transport [17]. For central ECRH deposition, the electron temperature increases, from around T e ≈ 200 eV in NBI up to T e ≈ 1.5 keV in ECRH and gets more peaked with decreasing density [32]. For these plasma parameters one expects bootstrap currents to be mainly driven by electrons and being only sizable in low density ECRH operation.…”

Section: Resultsmentioning

confidence: 99%