Double tearing mode in plasmas with anomalous electron viscosity

Abstract

The linear behavior of the double tearing mode in plasmas with a phenomenological anomalous electron viscosity is investigated within the framework of magnetohydrodynamic theory. In the large Reynolds number R=τv/τh (τv and τh are, respectively, the viscosity penetration time of the magnetic field and the Alfvén time for a plasma sheet of width a) limit, the growth rate is found to scale as R−1/5 if the two resonant surfaces, at x=±xs, are close enough to satisfy xs/a≪(kya)−11/15R−1/15. For larger separation between the resonant surfaces, the growth rate transits to a R−1/3 scaling. The transition occurs at xs/a∼(kya)−11/15R−1/15. The R−1/5 is shown to be closely correlated with the violation of the constant-ψ approximation. The nonlinear velocity perturbations associated with the unstable double tearing mode are estimated to saturate at a level high enough to serve as a trigger for the formation of transport barriers observed in advanced tokamaks.