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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. Yu^1)
 
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 B_t, is required for efficient stabilisation.

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