LINEAR FREQUENCY-RESPONSE OF RECONNECTING PERTURBATIONS

A crucial problem in the development of appropriate electrodynamic sys tems of feedback control of low-order nonlinear rotating resistive mod es in tokamaks consists of isolating, in the measurement of the Mirnov magnetic signals, the contribution due to the actual plasma perturbat ion from that due to the reflection of the controlling field. The fina l engineering objective is heavily dependent on the physical insight i nto the process of plasma response to the time-dependent controlling f ields with a helical pitch resonant to that of the closed lines of for ce of a magnetic surface with rational safety factor q. Accordingly, w e develop from first principles the theory of the time-harmonic respon se of a plasma to single helicity perturbations in the frame of the re duced magnetohydrodynamic (MHD) resistive model. The external field pe rturbation is split into an incident and a reflected component and an appropriate reflection frequency response function is evaluated. The r esults for modes of different helicities (m,n) are presented which can guide the construction of the compensation network of feeedback stabi lization systems of the type in use on the JET tokamak.