A crucial problem in the development of appropriate electrodynamic systems of feedback control of low-order nonlinear rotating resistive modes in tokamaks consists of isolating, in the measurement of the Mirnov magnetic signals, the contribution due to the actual plasma perturbation from that due to the reflection of the controlling field. The final engineering objective is heavily dependent on the physical insight into the process of plasma response to the time-dependent controlling fields with a helical pitch resonant to that of the closed lines of force of a magnetic surface with rational safety factor q. Accordingly, we develop from first principles the theory of the time-harmonic response of a plasma to single helicity perturbations in the frame of the reduced magnetohydrodynamic (MHD) resistive model. The external field perturbation is split into an incident and a reflected component and an appropriate reflection frequency response function is evaluated. The results for modes of different helicities are presented which can guide the construction of the compensation network of feeedback stabilization systems of the type in use on the JET tokamak.
Stabilizing effect of a longitudinal magnetic field on the evolution of helical plasma perturbations