Macroscopic dynamics, topology and transport barrier formation in Reversed Field Pinch plasmas

The reversed-field pinch configuration for the magnetic confinement of fusion plasmas is characterized by the emergence of self-organized quasi-helical states found both in magnetohydrodynamics (MHD) simulations and in high current RFP experiments. Numerical modeling can reproduce with high fidelity the MHD dynamics of high current RFP experiments, which are characterized by the formation of helical states, quasi-periodically relaxed to 3D states by strong reconnection events [1]. Seed helical perturbations at the magnetic boundary are expected to direct the plasma towards alternative helical states and first experimental tests have indeed confirmed this capability. Of particular interest is the possibility to stimulate non-resonant helical states, which feature better topological properties [2, 3]. Simulation results and first experimental evidence of alternative helical states formation in RFX-mod will be presented, together with a discussion of the different magnetic topology properties and their impact on transport barrier formation [4]. [1] Bonfiglio et. al. "Experimental-like Helical Self-Organization in Reversed-Field Pinch Modeling", PRL 111, 085002 (2013) [2] Veranda et. al. "Impact of helical boundary conditions on nonlinear 3D magnetohydrodynamic simulations of reversed-field pinch", PPCF 55 074015 (2013) [3] Veranda et. al. in preparation [4] Rubino et. al. "Detection of magnetic barriers in a chaotic domain: first application of Finite Time Lyapunov Exponent method to a magnetic confinement configuration", PPCF 57 085004 (2015)