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Title: Chapter 9: Performance-Limiting MHD Activity and Possibilities for Its Stabilization in ASDEX Upgrade

Journal Article · · Fusion Science and Technology
OSTI ID:20849586
;  [1]
  1. Max-Planck-Institut fuer Plasmaphysik (Germany)

Performance-limiting magnetohydrodynamic (MHD) instabilities on ASDEX Upgrade are discussed. In the conventional H-mode scenario, the main MHD performance limitation is found to be the neoclassical tearing mode (NTM). The onset {beta} of NTMs in ASDEX Upgrade scales with the poloidal ion gyroradius, in agreement with theoretical expectations. At higher {beta} values, NTMs occur in a more benign form, the frequently-interrupted-regime NTMs, which lead to a smaller confinement degradation than normal NTMs. Active control of NTMs by electron cyclotron current drive in the island has been demonstrated on ASDEX Upgrade. In advanced tokamak regimes with reversed shear, a variety of performance-limiting instabilities has been observed. The shear reversal zone can be unstable to double tearing modes or to infernal modes; both have been identified in ASDEX Upgrade. Due to the broad current profile in advanced tokamak discharges, the ideal external kink mode can be unstable at relatively low {beta}{sub N} {<=} 2; this is a main limitation to strongly reversed shear discharges with peaked pressure profiles. Finally, it is shown that fast-particle-driven modes such as fishbones can also have beneficial effects, such as providing stationary current profiles or triggering internal transport barriers.

OSTI ID:
20849586
Journal Information:
Fusion Science and Technology, Vol. 44, Issue 3; Other Information: Copyright (c) 2006 American Nuclear Society (ANS), United States, All rights reserved. http://epubs.ans.org/; Country of input: International Atomic Energy Agency (IAEA); ISSN 1536-1055
Country of Publication:
United States
Language:
English