Tearing mode stability with equilibrium flows in the reversed-field pinch

The influence of certain equilibrium flows on the stability of tearing modes in the reversed-field pinch is investigated. By solving the linearized magnetohydrodynamic equations in cylindrical geometry, the tearing mode stability factor Δ′ is calculated for a variety of axial flow profiles which have nonzero shear away from the rational surface, including flows localized entirely in the external, ideal region of the tearing mode. It is found that both m = 1 and m = 0 modes are destabilized by an axial flow localized near the edge of the plasma. This is the kind of flow that might be generated by any physical process creating an edge-localized radial electric field. A global flow profile with shear over the middle region of the plasma, simulating the differential rotation of core and edge modes observed in some reversed-field pinch discharges, is found to have a destabilizing effect on the m = 1 mode, while leaving the stability parameter of the m = 0 mode practically unchanged. The possible connection of these results with features of the spontaneous enhanced confinement regime in the Madison Symmetric Torus is discussed.