Transport characterization in chaotic magnetic fields through Lagrangian Coherent Structures

The magnetic confinement of fusion plasmas is sought in various configurations. One of them is the reversed-field pinch which is characterized by the emergence of self-organized quasi-helical states both in high current experiments and in magnetohydrodynamic (MHD) simulations. These simulations reproduce such states very well together with their quasi-periodic relaxation to 3D states involving a domain of chaotic magnetic field lines. This work presents the characterization of the transport-related properties of these 3D states. Lagrangian Coherent Structures in the chaotic domain are revealed by the Finite Time Lyapunov Exponent (FTLE) method as ridges of the FTLE field: ridges are defined as special lines whose points are locally at the highest point in the FTLE field transverse to the ridge. They are diagnosed to represent strong barriers for the magnetic field lines, which are effectively limited in their motion inside the chaotic sea. Quantitative information can be obtained by the analysis of the magnetic field line diffusion coefficient in a stochastic field using the NEMATO code for magnetic field lines tracing. This allows the optimization of low-transport helical states in the reversed-field pinch.