Active Control for Stabilization of Neoclassical Tearing Modes

We describe active control algorithms used by \hbox{DIII-D} to stabilize and maintain suppression of 3/2 or 2/1 neoclassical tearing modes (NTMs) using electron cyclotron current drive (ECCD) at the rational q-surface. The \hbox{DIII-D} NTM control system can determine the correct q-surface/ECCD alignment and stabilize existing modes within 100-200~ms of activation, or prevent mode growth with preemptive application of ECCD, in both cases enabling stable operation at normalized beta values above 3.5. Because NTMs can limit performance or cause plasma-terminating disruptions in tokamaks, their stabilization is essential to the high performance operation of ITER. The \hbox{DIII-D} NTM control system has demonstrated many elements of an eventual ITER solution, including general algorithms for robust detection of q-surface/ECCD alignment and for realtime maintenance of alignment following disappearance of the mode. This latter capability, unique to \hbox{DIII-D}, is based on realtime reconstruction of q-surface geometry by a Grad-Shafranov solver using external magnetics and internal motional Stark effect measurements. Alignment is achieved by varying either the plasma major radius (and the rational q-surface) or the toroidal field (and the deposition location). The requirement to achieve and maintain q-surface/ECCD alignment with an accuracy on the order of 1 cm is routinely met by the \hbox{DIII-D} Plasma Control System and these algorithms. We discuss the integrated plasma control design process used for developing these and other general control algorithms, which includes physics-based modeling and testing of the algorithm implementation against simulations of actuator and plasma responses. This systematic design/test method and modeling environment enabled successful mode suppression by the NTM control system upon first-time use in an experimental discharge.