Negotiating with helical magnetic self-organization, MHD predictions and Reversed Field Pinch experiments

Magnetic self-organization in toroidal pinches takes a peculiar helical shape when the ratio between toroidal current and toroidal magnetic flux (Pinch Parameter) is pushed above a characteristic threshold. This is clearly seen in the form of core kink instability in Tokamaks (so called Kruskal-Shafranov instability) and as a global helical shape in Reversed Field Pinch experiments (RFP). In this last case the nonlinear saturation approaching an ohmic helical equilibrium can be recognized. In fact, MHD modeling has been largely used to successfully capture such a basic phenomenon, quite evading the famous Taylor's theory for RFP relaxation. The globally helical RFP has been shown to bring interesting confinement properties, featuring internal transport barriers, which might be understood as a result of magnetic topological structures like Cantori sets or Lagrangian Coherent Structures, and magnetic chaos healing. Together with summarizing these general features, we here discuss the recent successful MHD prediction of alternative helical regimes, achievable by using edge magnetic perturbations with suitable choice of amplitude and helical pitch. In this way, typical features like helical amplitude and dithering frequency can be tuned almost at will, with impact on transport properties. A first set of RFX-mod experiments substantially confirms modeling predictions.