Effects of profiles on the neo-classical tearing mode

Numerical analysis of the island width of the neoclassical tearing mode, which is induced by the helical bootstrap current along the island, have been carried out to clarify effects of profiles on stability and to obtain conditions for stabilization by electron cyclotron current drive (ECCD). When the neoclassical tearing mode emerges, the saturated island grows up to some 10% of the minor radius at the beta value well below the ideal kink limit. The saturated island width of min mode can be small when the magnetic shear is large and the pressure gradient is low at the corresponding rational surface. Therefore, in the broad pressure and high ii (internal inductance) plasma, the neoclassical tearing mode is more stable when q(min) is slightly above one or two. In the broad pressure and low ii plasma, the dependence of the island width on q(0) is weak. A localized additional current at the O-point of the island by the ECCD significantly stabilizes the mode. The localization of the external current is effective, however, the stabilization also works well when the island width is smaller the radial width of the local current. The required current for the stabilization does not depend on the profiles very much. (C) 2001 Elsevier Science B.V. All rights reserved.