Control of resistive wall modes in a cylindrical tokamak with radial and poloidal magnetic field sensors

A simple cylindrical linear magnetohydrodynamic model is introduced to explain recent numerical results relating to stabilization of resistive wall modes by feedback. These older results indicate that feedback with sensors detecting the perturbed poloidal magnetic field is more effective than that with radial field sensors. The model used in this paper allows a complete analytic treatment, so that stability results for both radial and poloidal sensors and the effect of coils that couple poloidal harmonics are transparent. Results with radial sensors and with poloidal sensors, either inside or outside the resistive wall, are compared, showing that the results with internal poloidal sensors are indeed much better than those with radial sensors, if the coupling of poloidal modes by the coils is large. Results with external poloidal sensors are found to be comparable with those with radial sensors. The effect of a phase shift between sensor and control coils is investigated. Sensitivity of the three schemes to high frequency fluctuations, related to sensitivity to noise, is discussed. (C) 2004 American Institute of Physics.