Nonlinear, single-helicity magnetic reconnection in the reversed-field pinch

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Abstract
A computational and analytical study of the nonlinear evolution of single‐helicity, resistive, current‐driven modes in the reversed‐field pinch utilizing the single‐fluid magnetohydrodynamic approximation is presented. In the start‐up phase it is found that an off‐axis current peak can be anomalously dissipated by globally reconnecting current‐driven modes. The reconnection is seen to be well described by the Kadomtsev reconnection model, which is directly applied to the reversed‐field‐pinch regime. During the sustainment phase of the discharge, stable profiles are driven unstable by transport processes and restabilized by the resulting current‐driven instabilities. The dynamics of the toroidal field is of fundamental importance in the evolution of these modes. The reversed‐field‐pinch profile is restabilized by two successive global reconnections: the first removes the rational surface; the second restores it. It is also found that magnetic reconnection can occur in the reversed‐field pinch without a linear phase. Although the interaction of instabilities with different helicities is not directly investigated, the importance of this phenomenon in the reversed‐field pinch is discussed.