Electromagnetic modeling for the active control of MHD modes in RFX

The new RFX load assembly is characterized by a thin copper shell with a 50 ms time constant for the vertical field penetration, and 192 saddle coils, individually controlled, which cover the whole surface of the torus. The active control of Resistive Wall Modes is considered a basic issue to extend the pulse duration to its nominal value. Experimental results are expected to be of interest for Tokamak devices, as well. An open loop state space model was developed in view of the simultaneous control of different harmonic components of the magnetic field. It integrates a model of the saddle coil system and a linear model of the plasma MHD evolution. A lumped electromagnetic parameter representation of the saddle coil system was adopted and the coupling terms were calculated taking into account the presence of passive conductors such as vessel and shell. The plasma MHD behaviour is given in terms of the harmonic components, including the aliasing effects (“sidebands”) due to the “finite” number of actuators and sensors. The state space representation was chosen since it is particularly suited for the design of multivariable control system and is numerically more robust. Reduction techniques have also been tested in order to obtain lower order models for the control system design while retaining the basic static and dynamic properties.