Electric field and transport in W7-AS

Kick, M.; Maaßberg, H.; Anton, M.; Baldzuhn, J.; Endler, M.; Goerner, C.; Hirsch, M.; Weller, A.; Zoletnik, S.; X-Team, W

doi:10.1088/0741-3335/41/3a/048

Cited by 43 publications

(60 citation statements)

References 18 publications

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“…The neoclassically predicted ambipolar radial electric field is generally in agreement with observations in the core of CHS [26] and W7-AS [27,28,29,30] in the 'ion root' regime. Agreement has also been obtained in plasmas with net toroidal current and significant magnetic shear in W7-AS [31], and qualitative agreement has been reported in TJ-II [32].…”

Section: Neoclassical Transport In Stellarators: Theory and Experimentssupporting

confidence: 82%

“…The electron root would be especially valuable in reactors, since the ripple transport has strong temperature dependence. W7-AS experiments have confirmed that transport is reduced by either sign of E r [29]. The electron root is frequently predicted, but often not observed in low-density ECRH plasmas in W7-AS [29] and CHS [34].…”

Section: -9mentioning

confidence: 84%

“…W7-AS experiments have confirmed that transport is reduced by either sign of E r [29]. The electron root is frequently predicted, but often not observed in low-density ECRH plasmas in W7-AS [29] and CHS [34]. We adopt ion root solutions in Section 3 because we have no reason to expect NCSX to be in the electron root regime.…”

Section: -9mentioning

confidence: 99%

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Assessment of Transport in NCSX

Mikkelsen

Maaßberg

Zarnstorff

et al. 2007

Fusion Science and Technology

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We explore whether the energy confinement and planned heating in NCSX are sufficient to test MHD stability limits, and whether the configuration is sufficiently quasi-axisymmetric to reduce the neoclassical ripple transport to low levels, thereby allowing tokamak-like transport. A 0-D model with fixed profile shapes is related to global energy confinement scalings for stellarators and tokamaks; neoclassical transport properties are assessed with the DKES, NEO, and NCLASS codes; and a power balance code is used to predict temperature profiles. Reaching the NCSX goal of =4% at low collisionality will require H_ISS-95=3, which is higher than the best achieved in present stellarators. However, this level of confinement is actuallỹ 10% lower than that predicted by the ITER-97P tokamak L-mode scaling. By operating near the stellarator density limit, the required H_ISS-95 is reduced by 35%. The high degree of quasiaxisymmetry of the configuration and the self-consistent 'ambipolar' electric field reduce the neoclassical ripple transport to a small fraction of the neoclassical axisymmetric transport. A combination of neoclassical and anomalous transport models produces pressure profile shapes that are within the range of those used to study the MHD stability of NCSX. We find that =4% plasmas are 'neoclassically accessible', and are compatible with large levels of anomalous transport in the plasma periphery. 7-2

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Section: Neoclassical Transport In Stellarators: Theory and Experimentssupporting

confidence: 82%

Section: -9mentioning

confidence: 84%

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Assessment of Transport in NCSX

Mikkelsen

Maaßberg

Zarnstorff

et al. 2007

Fusion Science and Technology

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“…In the stronger ECR-heating plasmas of the Wendelstein-7AS, a centrally peaked profile of electron temperature is observed, and its large electron temperature gradient at a core radius suggests the transport barrier. 71 In contrast to tokamaks, the bifurcation mechanism of the improved confinement regime of the toroidal helical plasmas is simply ascribed to the neoclassical property of the radial electric field. This formation scenario was theoretically expected before its confirmation.…”

Section: Bifurcation and Transport Barriermentioning

confidence: 99%

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

et al. 2000

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The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System ͑CHS͒ ͓K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 ͑International Atomic Energy Agency, Vienna, 1989͒, Vol. 2, p. 411͔. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied.

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“…This is because the toroidal flow associated with the ambipolar radial electric field is small compared with the flow due to NBI in most L-mode discharges. In the hot electron mode where the electron thermal transport barrier is established, a large electric field ͑.10 kV͞m͒ appears in the plasma core [20][21][22], then toroidal flow associated with the ambipolar radial electric field may overcome the toroidal flow due to NBI. In this paper, the first observation of toroidal flow antiparallel to the ͗E r 3 B u ͘ drift direction (also antiparallel to the neutral beam injection) near the electron transport barrier in the hot electron mode plasmas is described.…”

mentioning

confidence: 99%

Observation of Toroidal Flow Antiparallel to the〈Er×Bθ〉Drift Direction in the Hot Electron Mode Plasmas in the Compact

et al. 2001

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A toroidal flow antiparallel to the ͗E r 3 B u ͘ drift direction is observed in the hot electron mode plasmas when a large positive electric field and a sharp electron temperature gradient are sustained inside the internal transport barrier in the Compact Helical System. This toroidal flow reaches up to 5 3 10 4 m͞s at the plasma center, and it is large enough to reverse the toroidal flow driven by a tangentially injected neutral beam. These observations clearly show the plasma favors flow in the minimum =B direction at the transport barrier. DOI: 10.1103/PhysRevLett.86.3040 PACS numbers: 52.55.HcA spontaneous poloidal flow has been recognized to be important in the confinement of toroidal plasmas, since poloidal flow velocity and radial electric field shear were found to contribute an improvement of plasma confinement in H-mode plasmas [1,2]. The turbulence in the plasma is predicted to be suppressed by a E 3 B velocity shear through the mechanism of a nonlinear decorrelation [3,4]. It has been confirmed experimentally that the E 3 B shearing rate exceeds the growth rate of the turbulence at the transport barrier [5][6][7]. The toroidal flow has been considered to be determined by the radial transport of momentum driven by neutral beam injection (NBI) with anomalous shear viscosity [8,9]. Although the E 3 B velocity shear due to the toroidal flow can be important in the plasma core [10], there have been few experiments that demonstrate a spontaneous toroidal flow not driven by NBI in tokamaks [11][12][13][14]. It is generally the case in tokamaks that the plasma rotates parallel (antiparallel) to the plasma current for the coinjected (counterinjected) NBI, which corresponds to a positive (negative) radial electric field ͑E r ͒ in L-mode plasmas [15]. However, when a negative E r is produced at the transport barrier, both localized toroidal flow in the counterdirection and localized poloidal flow in the electron diamagnetic direction are observed even for plasmas with coinjected NBI [5,16,17]. These results show the need for detailed measurements of both toroidal and poloidal flow at the transport barrier, where the toroidal flow profile is not determined by the transport of momentum driven by NBI alone. Yet, despite the importance of the coupling between toroidal and poloidal flow, no studies until now have discussed the toroidal flow at the internal transport, where there is a large radial electric field.In a Heliotron/Torsatron device, there is no toroidal symmetry, and the minimum =B trace has a helical structure and helical flow along the minimum =B can be expected. The significant difference between tokamaks and Heliotron devices is the relationship between the poloidal field direction and the direction of dominant symmetry.The pitch angle of dominant symmetry is even larger than that of the averaged poloidal field in a Heliotron device, while it is zero due to the toroidal symmetry in tokamaks. This helical flow should change sign depending on the sign of E r or on the direction of the magnetic field. H...

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Electric field and transport in W7-AS

Cited by 43 publications

References 18 publications

Assessment of Transport in NCSX

Assessment of Transport in NCSX

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

Observation of Toroidal Flow Antiparallel to the〈Er×Bθ〉Drift Direction in the Hot Electron Mode Plasmas in the Compact

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