The resistive wall mode (RWM) and neoclassical tearing mode (NTM) have been simultaneously suppressed in the DIII-D for durations over 2 seconds at beta values 20% above the no-wall limit with modest electron cyclotron current drive (ECCD) and low plasma rotation. The critical plasma rotation was significantly lower than reported at the IAEA FEC in 2006. However, even in this stabilized regime, stable steady-state operation is not unconditionally guaranteed. Various localized MHD activities such as edge localized modes (ELMs) and fishbones begin to couple to the RWM branch near the no-wall limit. Feedback is useful to improve the stability. Simultaneous operation of slow dynamic error field correction and fast feedback suppressed the ELM-induced RWM at high normalized beta. The result implies that successful feedback operation requires careful control of residual RWMs. The effectiveness of feedback operation was demonstrated using a reproducible current-driven RWM. The present findings are extremely useful in the challenge of control of RWM and NTM in the unexplored physics territory of burning plasmas in ITER.

The requirements of the DIII-D physics program have led to the development of many operational control results with direct relevance to ITER. These include new algorithms for robust and sustained stabilization of neoclassical tearing modes (NTM) with electron cyclotron current drive (ECCD), model-based controllers for stabilization of the resistive wall mode (RWM) in the presence of ELMs, coupled linear-nonlinear algorithms to provide good dynamic axisymmetric control while avoiding coil current limits, and adaptation of the DIII-D Plasma Control System (PCS) to operate next-generation superconducting tokamaks. Development of integrated plasma control, a systematic approach to model-based design and controller verification, has enabled successful experimental application of high reliability control algorithms requiring a minimum of machine operations time for testing and tuning. The DIII-D PCS hardware and software and its versions adapted for other devices can be connected to integrated plasma control simulations to confirm control function prior to experimental use. This capability has been important in control system implementation for tokamaks under construction and is expected to be critical for ITER.

Plasma Phys. Control. Fusion 52 (2010) 104004 Y In et al are based on a single mode assumption, so the investigation of the second least stable RWM is of high interest.

Active feedback control in the DIII-D tokamak has fully stabilized the current-driven ideal kink resistive-wall mode (RWM). While complete stabilization is known to require both low frequency error field correction (EFC) and high frequency feedback, unambiguous identification has been made about the distinctive role of each in a fully feedback-stabilized discharge. Specifically, the role of direct RWM feedback, which nullifies the RWM perturbation in a time scale faster than the mode growth time, cannot be replaced by low frequency EFC, which minimizes the lack of axisymmetry of external magnetic fields.