Onset and suppression of 2/1 NTM in DIII-D

Full text: Complete suppression of the important m = 2/n = 1 neoclassical tearing mode (NTM) has been achieved recently in DIII-D using electron cyclotron current drive (ECCD) in the island O-point. High performance, hybrid-regime discharges were used in these experiments with βN ∼ 2.8 (β ∼ 3.5%), which equals 90% of the ideal kink no wall stability limit. These NTMs were shown to be classically destabilized by varying dβ/dt prior to onset and comparing the resulting island growth rate to a comprehensive model, the latter which predicts maximum and minimum rates of heating for immediate NTM onset. Using five gyrotrons with 2.7 MW of unmodulated power to drive 40 kA of current at ρ = 0.66 (the location of the q = 2 surface), this NTM was stabilized resulting in a ∼ 30% improvement (recovery) of the energy confinement time and an increase in the toroidal rotation velocity. This demonstration of the complete suppression of the m = 2/n = 1 NTM using ECCD improves confidence that a control system to prevent confinement loss and disruptions arising from this mode can be developed in ITER. (author)