Abstract
Active feedback of non-axisymmetric external modes in tokamaks is studied by means of combined MHD and control analysis. The MARS code for toroidal MHD stability analysis has been extended to compute transfer functions for the electromagnetic response of the plasma, the coils and the resistive wall. These transfer functions are used for controller design. The controller is designed to allow for the longest possible response time of the amplifier-feedback coil circuit, such that certain predetermined performance criteria are satisfied. Calculations are presented for high beta advanced tokamak equilibria with ITER-FEAT shape.With an array of feedback coils that has only one coil in the poloidal direction, and assuming a single resistive wall, control of modes with toroidal mode number n = 1 is found to be possible for βN up to 5, or twice the no-wall limit. Sensors for the poloidal field are superior to sensors for the radial field. Feedback coils with a rather broad cross-section have significant advantages over thin-wire coils. The controller needs to have significant derivative action, but the requirements on the time constant of the amplifier-feedback coil circuit are moderate. This time needs to be less than a few resistive wall times, and broad strips allow an even longer time constant.
Published Version
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