(TH/P3-19) Modeling and Interpretation of MHD Active Control Experiments in RFX-mod

Abstract.  The RFX experiment is a Reverse Field Pinch (RFP) that has been modified (RFX-mod) by adding a set of 4(poloidally)×48(toroidally) feedback controlled saddle coils, which are able to accurately control the radial field at the conductive shell, providing a clean plasma magnetic boundary. This paper will present the results of a series of ongoing theoretical studies aiming at interpreting the experimental RFX-mod results. We successively consider Resistive Wall Modes (RWMs) active control and the interaction of an external applied field with the internal dynamo modes. The RWMs are slow MHD instabilities expected to play an important role in setting tokamaks beta limits and whose comprehension is therefore of key interest not only for RFPs. A linear response model is compared with the experimental data to interpret the observed stabilization. Other studies regard the so called Virtual Shell (VS) operation, where a zero normal magnetic field contour is created by using the large available number of active coils and sensors. This operation has been seen to affect both the non resonant modes (RWMs) and also the internal resonant tearing modes. A Newcomb solver constrained by the external measurements is employed to calculate the magnetic field components in the plasma region and to reconstruct particle and field line trajectories for transport studies. Finally a set of experiments have been carried out where a single helicity either resonant or non resonant is applied to the plasma by the external circuits. In the case of non resonant fields the phenomenon of Resonant Field Amplification (RFA) of marginally stable RWM has been studied by employing the linear response model, while for resonant fields a more complex interaction with the internal tearing modes is observed. This phenomenon is very interesting especially in the attempt of actively inducing the so called Single-Helical state and it is studied theoretically by employing 3D cylindrical nonlinear codes.

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