Structures in Reversed Field Pinch Magnetic Self-Organization, Insights and Prospects from 3D Nonlinear MHD

Since the early 90ties, 3D nonlinear MHD studies have been developing a fundamental framework for the understanding of the Reversed Field Pinch (RFP) helical self-organization. Indeed, this process began to manifest itself clearly only in late 90ties in high current experiments. Within 3D MHD, it was identified as a route to helical equilibrium, governed by viscoresistive dimensionless parameters, going beyond the well-known magnetic Taylor's magnetic relaxation theory application to the RFP. In this presentation, we will discuss the impact of self-organization on the transport properties, and the key role of boundary conditions (seed edge Magnetic Perturbations) in favoring the transition to the quasi-helical regimes. In particular, the transition leads to a significant reduction of the edge chain of 3D m~0 magnetic islands in favor of a macroscopic order characterized by a core helical structure. Seed edge Magnetic Perturbations -for the first time in advanced simulations- allowed for realistic description of the experimental quasi-helical regime, with remnant sawtoothing cycle, typical of medium current RFP discharges. In addition, it has been shown the capability of forcing various pitches of the helical plasma shape (with different character of the safety factor profile), leading to the discovery of new RFP helical regimes, whose transport properties are presently under investigation, and will be further experimentally studied in the RFX-mod2 experiment in Italy, which will start operation in 2024.