A model for the analysis of the magnetic field errors at the equatorial portholes of the RFX shell

A 3D electromagnetic finite element model has been developed, to reconstruct the magnetic field generated outside the plasma region by the tearing modes of a RFP plasma, in the presence of the vacuum vessel and the stabilizing shell. In particular, the model is applied to study the field error produced close to the shell equatorial gap, where a large number of holes for pumping and diagnostic accesses are present. Moreover it is used to check the feasibility of a system of correction coils to be installed outside the shell, in correspondence with those holes, in order to cancel, the field error. Finally, the effectiveness of the same coils in producing a rotating magnetic field, able to generate a dragging torque on the tearing modes, is assessed.