Magnetic reconnection, flow, and current density patterns in nonlinear magnetohydrodynamics modeling of reversed-field pinches and tokamaks

The typical sawtoothing activity observed in reversed-field pinch (RFP) configurations is studied in several respects, disclosing the nature of the 3D reconnection events as seen in nonlinear visco-resistive magnetohydrodynamic (MHD) modeling. We deal with quasi-stationary helical RFP regimes resulting from resistive-kink tearing modes activity, typical of RFP experiments. The nearly periodic crash can be described as the coalescence of contiguous windings of the helically deformed plasma current, bringing to intense localized reconnection patterns in regions featuring strong mode locking. Two consequences are well-known in the reconnection community: a conversion of magnetic into kinetic and thermal energy of the plasma, and the formation of steep current sheets. The velocity field reflects this behavior, as two outbursts with opposite directions are observed in clear relation with the collapsing helices. Significant electric fields are generated during this process, which may energize particles. A similar inspection is also extended to a case of forced magnetic reconnection in a schematic tokamak plasma, providing a paradigmatic example of the interaction between magnetic and velocity fields: it is shown that an external resonant magnetic perturbation can penetrate the plasma only if visco-resistive dissipation is above a threshold. We also observe that a sufficiently intense plasma axisymmetric flow can damp the externally imposed perturbation, and that the features of the plasma flow in such a simplified case are reminiscent of those observed in the nonlinear RFP reconnection process. Results support the value of our simplified nonlinear MHD modeling in providing insight into macroscopic behavior of current carrying plasma and favoring developments of new ideas to better confinement configurations.