Quasi-Single Helicity in the RFP: spontaneous emergence of order from chaos in a fusion plasma

Toroidal configurations for the magnetic confinement of fusion plasmas are divided in two large families: the first one is the pinch family, which includes the tokamak and the reversed field pinch (RFP), where magnetic field lines are twisted by the presence of a toroidal plasma current; the second one is the stellarator family, where the twist is given by a helical magnetic field component and the plasma is currentless. Both families display, to the lowest order, the symmetry required for avoiding magnetic chaos and ensuring good plasma confinement (toroidal and helical symmetry respectively). However, at low plasma current the RFP features the presence of a spectrum of helical perturbations, required for the sustainment of the configuration, which break this symmetry, so that magnetic chaos ensues. In the last few years it has been shown that at high plasma current a spontaneous transition occurs to a different state, called Quasi Single Helicity (QSH) state, where the perturbation spectrum becomes dominated by a single mode. This transition, which has been theoretically predicted by 3D MHD simulations, gives rise to a helically symmetric plasma, similar to those produced in stellarators. The QSH state features the presence of strong thermal gradients inside the plasma, so that the thermal content of the plasma core is increased. This contribution describes the main features of the QSH condition, showing that its duration increases with plasma current. It is also demonstrated that the thermal gradient inside the plasma is related to the reduction of the amplitude of the secondary modes, which results in a decrease of the magnetic chaos.