Feedback control of resistive wall modes in reversed field pinches

Feedback stabilization of magnetohydrodynamical modes in reversed field pinches is studied in cylindrical geometry, taking into account a finite number of coils, both in poloidal and toroidal directions. The robustness and stability of the feedback scheme is analysed by means of Nyquist diagrams for different arrangements of the feedback coils and magnetic sensors. The feedback can be adversely affected by the coupling of different Fourier components from the discrete coils, and configurations are sought where this coupling does not seriously degrade the performance. The requirements on the feedback system are discussed for different cases and low gain solutions are identified.