Stabilization of neoclassical tearing modes by localized ECCD in DIII - D

Neoclassical tearing modes (NTMs) are MHD modes which can become destabilized in a tokamak by a helical pressure perturbation. The NTM is particularly well suited to control since the mode is linearly stable although nonlinearly unstable, so if the island amplitude can be decreased below a threshold size the mode will decay and vanish. One means of shrinking the island is the replacement of the 'missing' bootstrap current by a localized current generated by electron cyclotron current drive (ECCD). This method has been applied to the m = 3/n = 2 and the 2/1 tearing modes in DIII-D, in H-mode plasmas with ongoing ELMs and sawteeth, both of which generate seed islands periodically. In the case of the 3/2 mode, full suppression was obtained robustly by applying about 1.5 MW of ECCD very near the flux surface of maximum mode amplitude. When the mode first appears in the plasma the stored energy decreases by 30%, but when the mode is stabilized by the ECCD the beta may be raised above the initial threshold pressure by 20% by additional neutral beam heating, thereby effecting an improvement in the limiting beta of nearly a factor 2. An innovative automated search algorithm was implemented to find and retain the optimum location for the ECCD in the presence of the mode. Only partial success has been obtained in stabilizing the 2/1 mode by ECCD, but calculations indicate that ECCD power near 3 MW should be adequate for complete suppression of this mode. (authors)