Reversed-field pinch pursuit of magnetic order exploiting helical states with transport barriers

Self-organized helical states are a ubiquitous feature inastrophysical and in current carrying hot magnetic-fusionplasmas. In the latter, self-organized helical states areobserved locally in the core of the tokamak - associatedwith strong 3D magnetohydrodynamics activity - andglobally in reversed-field pinch (RFP) plasmas. In RFPhelical states spontaneously develop from a dominantresistive-kink/tearing mode, creating a hot central regionbounded by transport barriers.In this contribution, we first report a milestone inmagnetohydrodynamics modeling of hot RFP plasmas [1,2] and experimental observation of helical states, namely,we show that experiments in RFX-mod confirm thenonlinear fluid modelling prediction of previouslyunobserved self-organized helical states. The keyelement is the use of a small edge magnetic perturbation[3, 4], with a helical periodicity different than thespontaneous dominant one observed in standarddischarges, which can convey its helical pitch to thewhole plasma column.As a second result, we find a higher magnetic order innumerical simulations where magnetic perturbations areapplied with a non-resonant helical twist. In such newstates we observe a broader zone where magnetic fieldlines lie on conserved magnetic surfaces and a reductionof magnetic field-lines transport with respect to standardregimes. Interestingly and counterintuitively, such newglobal helical regimes are obtained when exploitingnon-resonant Resistive Wall Modes, usually deemeddangerous for plasma discharges. First examples ofexperimental confirmation of such dynamical andtopological results are described.As a third result, we introduce a novel technique whichreveals confining structures, Cantori sets, in the shadowof the KAM tori [5]. We will also introduce preliminaryresults from the numerical solution of thetime-dependent, anisotropic temperature transportequation [6] applied to such new helical regimes comingfrom the MHD simulation codes.The novel possibility of changing the global helical twistof the plasma column, and therefore its safety factor,open new opportunities to deepen the theoreticalinterpretation of transport barrier formation andsustainment in hot-magnetic fusion plasma devices,including tokamaks and stellarators.