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 Phys. and Control. Fusion Research, Vol. 1 (International Atomic Energy Agency, Vienna, 1987) 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 toroidal field (and thus, 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 dB dt θ . 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 non-resonant 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".

The first suppression of the important and deleterious m=2/n= 1 neoclassical tearing mode (NTM) is reported using electron cyclotron current drive (ECCD) to replace the "missing" bootstrap current in the island 0-point. Experiments on the DIII-D tokamak verify that maximum shrinkage of the m=2/n=l island occurs when the ECCD location coincides with the q=2 surface. The DIII-D plasma control system is put into "search and suppress" mode to make small changes in the toroidal field to find and lock onto the optimum position, based on real time measurements of dBddt, for complete m=2/n=l NTM suppression by ECCD. The requirements on the ECCD for complete island suppression are well modeled by the modified Rutherford equation for the DIII-D plasma conditions. ...Recently several tokamaks have demonstrated the suppression of the m = 3/n = 2 NTM using unmodulated ECCD positioned at the island location. On ASDEX Upgrade, co-ECCD was verified to be more effective at NTM stabilization than counter-ECCD or pure heating alone [7-91. These experiments used a programmed sweep of the toroidal magnetic field to ensure that the current drive layer matched the mode location at some time during the ECCD pulse. On JT-GOU, suppression of the m = 3/n = 2 mode was also achieved for 1.5 s in steady conditions using ECCD

Two techniques were developed at DIII-D [J. L. Luxon, Nucl. Fusion 42, 64 (2002)] to tackle ITER-specific aspects of neoclassical tearing mode (NTM) control, namely, (1) the relatively small size of the rotating islands, smaller than the electron cyclotron current drive (ECCD) deposition region, and (2) the increased tendency of the islands, compared to present devices, to lock to the wall or to the residual error field, in a position not necessarily accessible to ECCD. Modulated ECCD is known to suppress small islands more efficiently, when “broad,” than continuous ECCD. At DIII-D, a NTM of poloidal/toroidal mode numbers m/n=3/2 was completely stabilized by a new technique where oblique electron cyclotron emission acted at the same time as an indicator of good alignment between ECCD and the island, and as a waveform generator, for modulation in synch and in phase with the island O-point. In another experiment, after locking in an unfavorable position, a 2/1 island was steered by externally generated magnetic perturbations, brought in the view of the gyrotrons and partly stabilized by ECCD in the island O-point. Magnetic perturbations were also used to sustain and control the mode rotation, which has the potential for an easier ECCD modulation.

The electron cyclotron emission (ECE) heterodyne radiometer diagnostic on DIII-D has been upgraded with the addition of eight channels for a total of 40. The new, higher frequency channels allow measurements of electron temperature into the magnetic axis in discharges at maximum field, 2.15 T. The complete set now extends over the full usable range of second harmonic emission frequencies at 2.0 T, covering radii from the outer edge inward to the location of third harmonic overlap on the high-field side. Full coverage permits the measurement of heat pulses and magnetohydrodynamic fluctuations on both sides of the magnetic axis. In addition, the symmetric measurements are used to fix the location of the magnetic axis in tokamak magnetic equilibrium reconstructions. Also, the new, higher frequency channels have been used to determine central Te with good time resolution in low-field, high-density discharges using third harmonic ECE in the optically gray and optically thick regimes.

No abstract