State-of-the-art neoclassical tearing mode control in DIII-D using real-time steerable electron cyclotron current drive launchers

Abstract

Real-time steerable electron cyclotron current drive (ECCD) has been demonstrated to reduce the power requirements and time needed to remove 3/2 and 2/1 neoclassical tearing modes (NTMs) in the DIII-D tokamak. In a world first demonstration of the techniques required in ITER, the island formation onset is detected automatically, gyrotrons are turned on and the real-time steerable ECCD launcher mirrors are moved promptly to drive current at the location of the islands. This shrinks and suppresses the modes well before saturation using real-time motional Stark effect constrained equilibria reconstruction with advanced feedback and search algorithms to target the deposition. In ITER, this method will reduce the ECCD energy requirement and so raise Q by keeping the EC system off when the NTM is not present. Further, in the experiments with accurate tracking of pre-emptive ECCD to resonant surfaces, both 3/2 and 2/1 modes are prevented from appearing with much lower ECCD peak power than required for removal of a saturated mode.