High-Confinement Regime at High βN Values due to a Changed Behaviour of the Neoclassical Tearing Modes

High confinement [with H = 1, ITERH-98( y, 2)] at β N 2.3 has been found in ASDEX Upgrade discharges with existing (m, n) = (3, 2) neoclassical tearing modes (NTMs). The reason for the high confinement at high β N valuesis a transition of the NTMs into the so-called frequently interrupted regime. In this regime, the NTM growth is frequently interrupted by sudden drops in amplitude. Because of the long NTM growth time, the resulting averaged NTM amplitude remains much smaller than the corresponding saturated value. As this behavior with the beneficial effect on energy confinement has been confirmed at JET, such a high confinement regime at β N > 2 might also be expected for ITER.