Geometrical implications of 3D fields in the reversed field pinch

Unlike the stellarator, current-carrying plasmas are particularly prone to sponta- neous deviations from the nominal equilibria. The details of the geometry - e.g., iota profile, metric coefficients, local shear - become partially unknown, depending on several simplifying assumptions in the MHD models. Neglecting the details can lead to inaccurate descriptions of the physical models, and/or to an erroneous interpretation of the measurements. In this work, we consider some geometrical aspects related to non-axisymmetric fields in toroidal configurations, focusing on the reversed field pinch. We review how non-axisymmetric fields are usually treated, and how they can be reconstructed by means of different approaches. The importance of dealing with a reliable geometry is shown with practical examples on transport analyses. On the other hand, we will see how some measurements can be correctly interpreted only if the topological features of a helical field are known, and similarly how the external action on the plasma, like the application of resonant fields, requires the knowledge of its basic symmetries in order to be effectively understood.