Increased stable beta in DIII-D by suppression of a neoclassical tearing mode using electron cyclotron current drive and active feedback

In DIII-D, the first real-time active control of the electron cyclotron current drive stabilization of a neoclassical tearing mode (here m/n=3/2) is demonstrated. The plasma control system is put into a 'search and suppress' mode to make either small rigid radial position shifts (of order 1 cm) of the entire plasma (and thus the island) or small changes in toroidal field (of order 0.5%) which radially moves the second harmonic resonance location (and thus the rf current drive). The optimum position minimizes the real-time mode amplitude signal. Stabilization occurs despite changes in island location from discharge-to-discharge or from time-to-time. The neutral beam heating power is then programmed to rise after mode suppression by the ECCD. The plasma pressure increases higher than the peak at the onset of the neoclassical tearing mode until the magnetic island reappears. Real-time tracking of the change in location of q=3/2 due to the Shafranov shift with increasing beta is necessary to keep the ECCD at the optimum location in the absence of a mode. (author)