EX/P8-11 1 Experiments on feedback control of multiple resistive wall modes comparing different active coil arrays and sensor types J.R. Drake 1 , P.R. Brunsell 1 , D. Yadikin 1 , H. Bergsåker 1 , M. Cecconello 1 , A. Hedqvist 2 , M. Kuldkepp 2 , S. Menmuir 2 , E. Rachlew 2 1) Division of Fusion Plasma Physics, Alfvén Laboratory, Association EURATOM-VR, School of Electrical Engineering, Royal Institute of Technology KTH, Stockholm, Sweden 2) Department of Physics, Association EURATOM-VR, School of Engineering Science, Royal Institute of Technology KTH, Stockholm, Sweden e-mail contact of main author: james.drake@ee.kth.se Abstract. Experiments have been carried out on the EXTRAP T2R reversed-field pinch device to study several important issues related to feedback control of resistive wall modes (RWMs). The feedback system includes a sensor coil array, a feedback controller implementing a feedback law and an active coil array. The issues include 1) effects of sideband harmonics produced by the feedback system, 2) the form of the controller and the feedback law, 3) feedback system stability, 4) selection of the sensor coil configuration and 5) effects of field errors on the feedback system. Side band harmonics are produced by the feedback system because the active saddle coil array consists of discrete coils. The presence of side bands can couple modes thus preventing simultaneous stabilisation of the coupled modes. The side band effect sets requirements for the minimum number of active coils in the array in both the poloidal and toroidal directions. Recent experiments using the intelligent shell concept with proportional-integral-derivative controller action have achieved complete simultaneous stabilisation of all RWMs modes when the requirements are satisfied. In addition to the intelligent shell concept, preliminary experiments have been performed to test the fake rotating shell concept. For this concept, the sensor coil array is shifted in phase relative to the active coil array thus a detected harmonic is induced to rotate by the active coil-produced control field. Under the condition that the phase shift is less than a quarter-wave length of the mode, mode suppression can be achieved. Feedback using a controller incorporating individual mode control has also been tested. This has enabled the first feedback experiments using a sensor array measuring the toroidal field component to be carried out. For this concept, an array consisting of localised toroidal field sensor coils is used. Mode suppression has been successfully accomplished. However pick-up of high order field error harmonics due to the small size of the sensor coils introduces an adverse signal to background ratio as compared to the case with the radial field sensor coil array. Optimal suppression is achieved at the predicted complex feedback gain phase. Mode rotation is induced at other complex gain phases, in agreement with modelling. In other experiments, linear models have been used to evaluate the effect of resonant field errors on mode growth. The thin-wall model is satisfactory for describing the response of the plasma and resistive-shell system and resonant field error amplification is observed. 1. Introduction Feedback control of resistive wall modes is of common interest for toroidal fusion concepts that use conducting walls for stabilization of ideal MHD modes. According to the thin-wall model if the wall is resistive and lies within the position for ideal wall stabilization, a class of modes called resistive wall modes (RWMs) develop with growth rates of the order of the wall penetration time [1]. For the advanced tokamak, control of modes with low toroidal mode number n is of primary interest for achieving beta above the no-wall limit (RWMs are n ≠ 0 pressure-driven external kinks). For the reversed-field pinch (RFP), there are unstable modes with poloidal mode number m = 1 with a range of toroidal mode numbers which must be simultaneously controlled. The commonly used feedback control system includes a sensor consisting of an array of magnetic coils (measuring mode harmonics), an actuator consisting of an array of active saddle coils (producing control harmonics) and a controller that implements a feedback law in real time. Active feedback control of RWMs has previously been demonstrated on the EXTRAP T2R reversed-field pinch experiment [2-4]. These experiments have shown that,