ONSET AND SUPPRESSION OF 2/1 NTM IN DIII-D

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 with βN ≈ 2.8 (β ≈ 3.5%), which equals 90% of the ideal kink no-wall stability limit. Using up to 3.0 MW of unmodulated power to drive up to 40 kA of ECCD at ρ = 0.67 (the location of the q = 2 surface), the m = 2/n = 1 NTM was stabilized resulting in a ≈30% improvement (recovery) of the energy confinement time. Recent developments in the theory of NTMs have enabled us to identify the observed islands as being classically destabilized with ′ ∆ > 0. This was done in experiments by varying the heating rate dβ/dt prior to onset and comparing the resulting island growth rate to a comprehensive model. The time dependence of ′ ∆ as β increased was a critical physics element along with neoclassical and polarization effects. The theory predicts transitions at different heating rates for onset of classical modes, seeded NTMs, and ideal modes. These were in agreement with the modes observed.