Resistive wall mode dynamics and active feedback control in DIII-D

Recent DIII-D experiments have shown that the growth of the n = 1 resistive wall mode (RWM) can be influenced by an external magnetic field applied in closed loop feedback using a six element error field correction coil (C coil). The RWM constitutes the primary limitation to normalized beta in recent DIII-D advanced tokamak plasma experiments. The toroidal rotation of DIII-D plasmas does not seem sufficient to completely suppress the RWM: a very slowly growing (growth rate γ<<1/τw) or saturated RWM is often observed at normalized beta above the no wall limit and this small RWM slows the rotation. As the rotation decreases, there is a transition to more rapid growth (γ~1/τw, where τw is the resistive time constant of the wall). The application of magnetic feedback is able to hold the RWM to a very small amplitude, prolonging the plasma duration above the no wall limit for times much longer than τw. These initial experimental results are being used to compare control algorithms, to benchmark models of the feedback stabilization process and to guide the design of an upgraded coil sensor system for stabilization of the RWM at normalized beta values closer to the ideal wall limit.