Effects of control voltage saturation and sensor noise on the resistive wall mode feedback in ITER

Active control of the n = 1 (n is the toroidal mode number) resistive wall mode (RWM) is numerically studied for an ITER 9 MA plasma designed for the advanced scenario. The large L/R response time of the active coils in ITER results in a significant reduction of the open-loop RWM growth rate, when the active coils act as passive conductors. For a typical RWM, the linear flux-to-voltage control scheme, with proportional controller only, yields complex closed loop eigenvalue (in the absence of plasma flow and drift kinetic effects), before the mode is fully stabilized by feedback. Without sensor signal noise, the RWM feedback system can tolerate a low level of control voltage saturation, typically in the order of 1 V in ITER. The presence of high-frequency sensor signal noise, however, can significantly increase the tolerable level of control power saturation. For a plasma close to the ITER target, and with the feedback gain well beyond the critical value for the linear closed loop stability, the tolerable voltage saturation level is predicted to be about 4V at the sensor signal noise level (standard deviation) of 0.25 G, and about 40V at noise level of I G. A proportional-derivative controller is beneficial for ideal linear control systems as well in systems with voltage limitation. In the presence of both voltage limitation and sensor noise, the derivative action may make it more difficult to be stabilize the RWM in ITER.