Linear and nonlinear thresholds of neoclassical tearing modes in tokamaks

The understanding of the physics of neoclassical tearing modes (NTM) is important for the dimensioning of current drive systems used to stabilize these modes in International Thermonuclear Experimental Reactor (ITER) plasmas [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)]. Predictions by theoretical models for the dynamics of NTM’s are compared with full scale numerical magnetohydrodynamical simulations including bootstrap current and transport effects. It is shown that curvature currents are sufficient to generate a nonlinear stability threshold for NTM’s. Furthermore, it is emphasized that at large resistivity NTM’s behave as an ordinary linear instability, which suppresses this nonlinear stability threshold.