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(EX3/1) The Physics of Neoclassical Tearing Modes and their Stabilisation by ECCD in ASDEX Upgrade

H. Zohm1), G. Gantenbein2), A. Gude1), S. Günter1), F. Leuterer1), M. Maraschek1), J. Meskat2), W. Suttrop1), Q. Yu1)
 
1) MPI für Plasmaphysik, D-85748 Garching, EURATOM Association
2) IPF Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart

Abstract.  The physics understanding of Neoclassical Tearing Modes (NTMs) gained through experiments and modelling at ASDEX Upgrade is presented. The onset $ \beta_{N}^{}$ for NTM is found to be proportional to the normalised ion gyroradius $ \rho^{*}_{}$ and independent of the normalised collisionality $ \nu^{*}_{}$ for a wide range of $ \nu^{*}_{}$. This scaling is in accordance with both polarisation current and $ \chi_{\perp}^{}$/$ \chi_{\Vert}^{}$ model, if for the latter, the heat flux limit on parallel heat conductivity is taken into account. Analysis of the structure and dynamics of NTMs validates the negative $ \Delta{^\prime}$. Complete stabilisation using ECCD has been demonstrated at $ \beta_{N}^{}$ = 2.5 and with about 10 % of the total heating power. The results are in good qualitative agreement with modelling using the Rutherford equation and in quantitative agreement with a 2 dimensional nonlinear cylindrical tearing mode code. A precise positioning of the ECCD microwave beam, so far achieved by feed-forward variation of Bt, is required for efficient stabilisation.

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IAEA 2001