<i>H</i>mode of the W 7-AS stellarator

Erckmann, V.; Wagner, F.; Baldzuhn, J.; Brakel, R.; Burhenn, R.; Gasparino, U.; Grigull, P.; Hartfuß, H. J.; Hofmann, J.; Jaenicke, R.; Niedermeyer, H.; Ohlendorf, W.; Rudyj, A.; Weller, A.; Bogdanov, S. D.; Bomba, B.; Borschegovsky, A. A.; Cattanei, G.; Dodhy, A.; Dorst, D.; Elsner, A.; Endler, M.; Geist, T.; Giannone, L.; Hacker, H.; Heinrich, O.; Herre, G.; Hildebrandt, D.; Hiznyak, V. I.; Ильин, В. И.; Kasparek, W.; Karger, F.; Kick, M.; Kubo, S.; Kuftin, A. N.; Kurbatov, V.I.; Lazaros, A.; Malygin, S. A.; Малыгин, В. И.; McCormick, K.; Müller, G.; Orlov, V.B.; Pech, P.; Ringler, H.; Roi, I. N.; Sardei, F.; Sattler, Sebastian; Schneider, F.; Schneider, Ursula; Schüller, P. G.; Siller, G.; Stroth, U.; Tutter, M.; Unger, E.; Wolff, H.; Würsching, E.; Zöpfel, S.

doi:10.1103/physrevlett.70.2086

Cited by 149 publications

(47 citation statements)

References 19 publications

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“…18,19 Before these findings, bifurcation of the radial electric field had been expected in toroidal helical plasmas by a neoclassical theory including helical ripple diffusion. [20][21][22] In contrast to tokamaks, the neoclassical theory had predicted that the absolute value of the radial electric field affected the transport in toroidal helical plasmas.…”

Section: Introductionmentioning

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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Section: Introductionmentioning

confidence: 99%

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

et al. 2000

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“…Clear evidence of further confinement improvement has also been observed. [4][5][6] However, the enhancement factor of confinement has been smaller than in large tokamaks. The reasons can be attributed to connection of the boundary where neutrals and atomic processes play an essential role and the core where high-temperature plasmas are contained.…”

Section: 2mentioning

confidence: 99%

Initial physics achievements of large helical device experiments

et al. 1999

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have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas ͑Rϭ3.9 m, aϭ0.6 m͒, which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection ͑NBI͒ has produced plasmas with a fusion triple product of 8ϫ10 18 keV m Ϫ3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to ͗␤͘ϭ0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics ͑MHD͒ instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.

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“…The enhanced confinement is attributed to an edge temperature pedestal [12,13,14], which is not generally observed in other stellarators but is not attributed to an H-mode in LHD. The pedestal may be associated with a chain of magnetic islands at or near the plasma edge [12], while avoidance of island chains at low order rational i values is associated with H-modes in W7-AS [15,16].…”

Section: -5mentioning

confidence: 99%

“…In stellarator H-modes the energy confinement is enhanced modestly (no more than 30%) in W7-AS [15,16] and CHS [53,54,55,56]. Access is typically restricted to narrow ranges ini that minimize damping of poloidal rotation at the plasma boundary by avoiding island chains [57,58] (in W7-AS toroidal rotation is strongly damped by toroidal mirrors, but poloidal rotation is possible).…”

Section: Enhanced Confinement Regimes In Stellaratorsmentioning

confidence: 99%

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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Hmode of the W 7-AS stellarator

Cited by 149 publications

References 19 publications

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

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

Initial physics achievements of large helical device experiments

Assessment of Transport in NCSX

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