Control analysis for non-axisymmetric resistive wall mode stabilization in the DIII-D tokamak

Control schemes and sensor types for the nonaxisymmetric resistive wall mode (RWM) stabilization in the DIII-D tokamak are compared. The analysis is based on a system response model for the RWMs in DIII-D and the modelling uses a toroidal current sheet to represent the plasma surface and a combination of helical and circulating "picture frame" currents to represent the conducting structure. The instability driving term and corresponding growth rate are derived from specification of the critical wall distance beyond which the mode will be magneto hydro dynamic unstable. Based on this model, control analysis is applied to investigate achievable closed loop behavior in terms of damping, robustness and control activity. The results show that with reasonable control signals, the RWMs can be stabilized. In addition, with sensors inside the wall, rather than outside, twice as good robustness towards variation in the RWM growth rate is achieved.