Control of neoclassical tearing modes in DIII–D

The development of techniques for neoclassical tearing mode (NTM) suppression or avoidance is crucial for successful high beta/high confinement tokamaks. Neoclassical tearing modes are islands destabilized and maintained by a helically perturbed bootstrap current and represent a significant limit to performance at higher poloidal beta. The confinement-degrading islands can be reduced or completely suppressed by precisely replacing the “missing” bootstrap current in the island O-point or by interfering with the fundamental helical harmonic of the pressure. Implementation of such techniques is being studied in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] in the presence of periodic q=1 sawtooth instabilities, a reactor relevant regime. Radially localized off-axis electron cyclotron current drive (ECCD) must be precisely located on the island. In DIII-D the plasma control system is put into a “search and suppress” mode to make either small rigid radial position shifts of the entire plasma (and thus the island) or small changes in the toroidal field (and, thus, the ECCD location) to find and lock onto the optimum position for complete island suppression by ECCD. This is based on real-time measurements of an m/n=3/2 mode amplitude dBθ/dt. The experiment represents the first use of active feedback control to provide continuous, precise positioning. An alternative to ECCD makes use of the six toroidal section “C-Coil” on DIII-D to provide a large nonresonant static m=1, n=3 helical field to interfere with the fundamental harmonic of an m/n=3/2 NTM. While experiments show success in inhibiting the NTM if a large enough n=3 field is applied before the island onset, there is a considerable plasma rotation decrease due to n=3 “ripple.”