(IT/P1-13) Optimization of Sensor Signals for Resistive Wall Mode Control in ITER

Y.Q. Liu1), J.B. Lister2)
1) Chalmers University of Technology, Gothenburg, Sweden
2) Ecole Polytechnique Federale de Lausanne (EPFL), CRPP, CH-1015 Lausanne, Switzerland

Abstract.  Advanced tokamak scenarios, such as the ITER Scenario-4, may suffer from global ideal MHD instabilities - the low-n, non-axisymmetric resistive wall modes (RWM), which limit the operational space in terms of achievable plasma pressures under steady state operation. It is highly desirable to stabilize the RWM in ITER, especially for the n=1 mode that gives the most severe pressure limit for the Scenario-4. Our previous work has indicated that, since rotational stabilization of the n=1 RWM may not be robust in ITER, active control of this mode is necessary. Some recent work for the vertical stability control (the n=0 RWM control) has suggested an idea of using a combined sensor signal, measured at various poloidal locations, as one way to improve the feedback system. The key element in this idea is to use as many as possible sensor signals, in order to extract the response purely from the full system unstable mode. In this work, we apply the similar idea to improve the n=1 RWM control in ITER using radial sensors. The additional benefit of extracting purely unstable RWM is that, (in ideal case) only proportional feedback gains are required to stabilize the mode. No derivative gains are needed even at high plasma pressures. This is highly desirable since derivative gains require the time differentiation of sensor signals, which increases the noise level in the sensor signal. We use the single fluid MHD stability code MARS-F for the study of the RWM feedback stabilization. The plasma pressure in ITER is characterized by a parameter Cβ, which scales linearly with the plasma pressure, such that Cβ = 0 corresponds to the no-wall pressure limit, and Cβ = 1 corresponds to the pressure limit with an ideal wall. Our calculations show that, by keeping the present design of the control coils in ITER (the super-conducting side correction saddle coils), and by using three sets of radial sensors along the poloidal angle, it is possible to find a single, optimal linear combination of the sensor signals, which leads to stabilization of the n=1 RWM for Cβ up to about 0.9. The stabilization is achieved by using proportional feedback only. For comparison, using the mid-plane radial sensors alone stabilizes the mode for Cβ up to about 0.4.

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