Electron temperature barriers as Lagrangian Coherent Structures

The formation of transport barriers with impact on transport and dynamical regimes is common to the main configurations for the magnetic confinement of hot plasmas: tokamak, stellarator, and reversed-field pinch (RFP). The aim of this work is to describe and deploy a numerical method for computing "Lagrangian Coherent Structures" (LCS) in scenarios typical of RFP helical self-organization, where magnetic stochasticity and electron temperature barriers appear together. LCSs are coherent curves, developed and studied in the framework of dynamical systems, constructed as the most attractive or repelling of nearby magnetic field lines. As a result, LCSs have a significant impact on the transport of physical quantities. In this work, LCSs are shown to behave as barriers to magnetic field lines in chaotic regions of numerically verified 3D nonlinear MHD simulations of RFP and in experimental data from the RFX-mod experiment in Padua. In the first case, LCSs locate / divide regions with different connection length. In experiments, LCS coincide with high electron temperature gradients, showing that transport barriers are akin to simple objects of dynamical system theory called Cantori.