Global Alfvénic modes excitation in ohmic tokamak plasmas following magnetic reconnection events

Abstract

A possible triggering mechanism of Alfvén waves (AWs) in tokamak plasmas, based on localized perturbations induced by magnetic reconnection events, is discussed in the framework of nonlinear viscoresistive 3D magnetohydrodynamics (MHD) modeling. Numerical simulations are performed with the SpeCyl code (Cappello and Biskamp 1996 Nucl. Fusion 36 571) that solves the equations of the viscoresistive MHD model in cylindrical geometry. We investigate a ohmic tokamak configuration where the m = 1, n = 1 internal kink mode (m is the poloidal mode number and n is the toroidal mode number) undergoes a complete reconnection process. An in-depth investigation of the process shows a spatio-temporal correlation between the velocity perturbations associated with the reconnection and the excitation of the shear AW in the core region and the global Alfvén eigenmodes, both with dominant m = 1, n = 0 periodicity. In particular they are observed to emanate from the outflow cones of the reconnection layer associated with the internal kink. The excitation mechanism described in this paper could explain the observations of Alfvénic fluctuations in the absence of energetic ions in several tokamak experiments documented in the literature and could contribute to AWs excitation in general, even in the presence of fast particles. This result shares similarities with analogous study in reversed-field pinch (RFP) configuration (Kryzhanovskyy et al 2022 Nucl. Fusion 62 086019) where AWs were found to be excited by the RFP sawtoothing.