Neoclassical tearing modes and their stabilization by electron cyclotron current drive in ASDEX Upgrade

Neoclassical tearing modes (NTMs) are studied in the ASDEX Upgrade tokamak [O. Gruber, H.-S. Bosch, S. Gunter , Nucl. Fusion 39, 1321 (1999)]. The onset beta (N) for NTMs scales with the normalized ion gyroradius rho*, independent of the normalized collisionality nu* for a wide range of nu (*). This scaling is in accordance with both polarization current and chi (perpendicular to)/chi (parallel to) model, if for the latter, the heat flux limit on parallel heat conductivity is taken into account. Analysis of NTM stabilization experiments indicates that the typical seed island size is 1-1.5 cm, again in agreement with both models. Mode coupling to the q=1 surface is a crucial element in determining the impact of the NTM on the discharge performance. Complete stabilization using electron cyclotron current drive (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 modeling using the Rutherford equation and in quantitative agreement with a two-dimensional nonlinear cylindrical tearing mode code. A precise positioning of the ECCD microwave beam, so far achieved by feedforward variation of B-t, is required for efficient stabilization. This calls for feedback control of the stabilization scheme. Our plans to implement such a scheme on ASDEX Upgrade are discussed. (C) 2001 American Institute of Physics.