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 [1] and in nonlinear magnetofluid numerical simulations [2]. We show the 3D nonlinear MHD prediction that small edge helical magnetic perturbations can shape the whole plasma column with their helical twist [3], 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 [4], and can explain the observation of electron temperature transport barriers in experiments [5]. 1 - R.Lorenzini, E.Martines, et al, Nature Physics \textbf{5}, 570 (2009) 2 - D.Bonfiglio, M.Veranda, et al, PRL \textbf{111}, 085002 (2013) 3 - M.Veranda, D.Bonfiglio, et al, PPCF \textbf{55}, 074015 (2013) 4 - G.Rubino, D.Borgogno, et al, NF \textbf{57}, 085004 (2015) 5 - M.Gobbin, D.Bonfiglio, et al, PRL \textbf{106}, 025001 (2011)