Simplified magnetic circuit for the calculation of the stray magnetic flux through the shell gaps

Significant toroidal magnetic field perturbations, stray flux at the shell gaps and current mismatching in the coils of the toroidal field winding are measured during the start-up and the flat-top phases of RFX. These phenomena are consistent with large and wall locked MHD modes: at first some m=1 modes evolve separately one after the other, afterwards they concur to a wide and localised plasma perturbation that persists during the flat-top. These perturbations are heavily influenced by the stray magnetic flux through the shell gaps. Hence a magnetic circuit that mainly considers the magnetic reluctances of the conducting shell gaps has been developed in order to estimate this stray flux and, therefore, to evaluate the stabilising capability of the shell. The observation of the MHD modes, the description of the equivalent magnetic network, the estimation of the stray flux and the comparison with the experimental measurements are reported