Helical self-organization in the reversed-field pinch: detection of barriers to transport

Self-organized helical states are an ubiquitous feature in astrophysical and magnetic fusion plasmas. In the reversed-field pinch plasmas quasi-helical states are observed both in high current experiments and in nonlinear magnetofluid numerical simulations. We show the 3D nonlinear MHD prediction that small edge helical magnetic perturbations can shape the whole plasma column with their helical twist, a result confirmed by dedicated experiments in RFX-mod. When a non-resonant twist is imposed chaos healing results in a higher area of conserved magnetic surfaces and in reduced diffusion of magnetic field lines. Barriers to the transport of magnetic field lines are detected: they can be identified as cantori of the magnetic field, and can explain the observation of electron temperature transport barriers in experiments.