Bifurcation of magnetic island saturation controlled by plasma viscosity
Résumé
Two nonlinear regimes, depending on the magnetic Prandtl number Prm, are identified for magnetic islands described by resistive MHD equations. The frontier between these two regimes is sharp, and has the characteristics of a phase transition controlled by plasma viscosity. In the low Prm regime, a new form of the so-called flip instability, consisting of a sudden change of the island phase, is identified. Already known in the context of a forcing by external magnetic perturbations and localized current drive, it occurs spontaneously at low Prm. The main characteristics of this new structural instability are described. The low Prm regime is well described by the slab visco-resistive model in the linear phase, and is characterized by both a large saturation of the island and strong nonlinearly driven zonal flows (that do not impact significantly the island dynamics however), while curvature physics strongly impacts the viscous regime.
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