STABILIZATION OF NEOCLASSICAL TEARING MODES IN TOKAMAKS BY ELECTRON CYCLOTRON CURRENT DRIVE

Resistive neoclassical tearing modes (NTMs) are anticipated to be the principal limit on stability and performance in ITER as the resulting islands break up the magnetic surfaces confining the plasma. Drag from island-induced eddy currents in the resistive wall can slow plasma rotation, produce locking to the wall, and cause loss of the high-confinement H-mode and disruption. NTMs are destabilized by helical perturbations to the pressure-gradient-driven "bootstrap" current. NTMs can be stabilized by applying co-electron-cyclotron current drive (ECCD) at the island rational surface. Such stabilization and/or preemption is successful in ASDEX Upgrade, DIII-D, and JT-60U, if the peak off-axis current density is comparable to the local bootstrap current density and well-aligned.

ASDEX Upgrade has used a feed-forward sweep of the toroidal field to get ECCD alignment on the island. JT-60U has used feed-forward sweeps of the launching mirror for the same purpose, followed up by real-time adjustment of the mirror using the electron cyclotron emission (ECE) diagnostic to locate the island rational surface. In DIII-D, ECCD alignment techniques include applying "search and suppress" real-time control to find and lock onto optimum alignment (adjusting the field or shifting the plasma major radius in equivalent small steps).

Most experimental work to date uses narrow, cw ECCD; the relatively wide ECCD in ITER may be less effective if it is also cw: the stabilization effect of replacing the "missing" bootstrap current on the island O-point could be nearly cancelled by the destabilization effect on the island X-point if the ECCD is very broad. Modulating the ECCD so that it is absorbed only on the m/n = 3/2 rotating island O-point is proving successful in recovering ECCD effectiveness in ASDEX Upgrade when the ECCD is configured for wider deposition.

The ECCD in ITER is relatively broad, with current deposition full width half maximum almost twice the marginal island width. This places strict requirements on ECCD alignment, with the cw effectiveness dropping to zero for misalignments as small as 2 cm. Tolerances for misalignment are presented to establish criteria for the alignment by moving mirrors in ITER for both cw and modulated ECCD.