Transport processes in chaotic magnetic field configuration: the Quasi-Single-Helicity states in RFX-mod

Magnetic field lines embedded in a plasma confinement system are often characterized by a chaotic motion, that can lead to the degradation of the confinement properties of the system. However, even in case of chaotic domains, magnetic barriers can emerge and limit the field line motion itself. Recently a fluid dynamical approach for the detection of these barriers hase been developed [1, 2]. The method relies on the behavior of the Finte Time Lyapunov Exponent (FTLE) field, associated to the magnetic configuration. This FTLE field shows the presence of ridges, that can be recognized as Lagrangian Coherent Structures (LCSs) for the magnetic field, and act as barriers for the transport of magnetic field lines across them. Here we provide the first application of this method to a real magnetic configuration [3]. We consider the magnetic field characterizing the emergence of self-organized quasi-helical states [4], predicted by magnetohydrodynamic (MHD) simulations [5] and observed in high current experiments in the reversed field pinch configurations. We show that the LCSs we find actually limit the field lines motion and provide a chaos healing effect, by influencing the transport properties of the system.