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

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.