LINEAR STABILITY ANALYSIS OF FORCE-FREE EQUILIBRIA LEADING TO QUASI-SINGLE-HELICITY STATES

An innovative argument is presented in order to explain the formation of Quasi-Single-Helicity (QSH) states in Reversed-Field-Pinches (RFPs) as result of a tearing perturbation of a force-free equilibrium. In particular it is shown that force-free equilibria with a piecewise constant ratio between the current density and the magnetic field can be tearing unstable to modes with helicity corresponding to the one observed during QSH states, whereas they are stable with respect to modes with other helicities. It is suggested that RFPs could reach such equilibria as a consequence of an evolution of the system from a relaxed Taylor state toward a non-reversed force-free state on resistive time scales.