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Cascades of high-n tearing modes with mode bursts in order of increasing n number have recently been observed in ASDEX Upgrade in the vicinity of the q = 1 surface. The current paper is a more detailed description of these investigations. The relationship between the poloidal and toroidal mode numbers is m = n + 1. A cascade commences with a low n number, typically 5 +/- 1, and can reach values around 22. They are observed in discharges with impurity accumulation and with a transient evolution of the q profile, where the magnetic shear is low and the resistivity high. Calculations with the CASTOR code have shown that high-n tearing modes become unstable if the magnetic shear is low, resistivity is high and the pressure gradient high enough to make mode coupling important. All three conditions are needed to obtain a positive and large growth rate. These CASROR code calculations are consistent with the experimental observations.

Additional information

Authors: GUDE A ET AL, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Article: Plasma Physics and Controlled Fusion, Vol. 40 (1998) pp. 1057-1071
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