Analysis of phase locking of tearing modes in reversed field pinch plasmas

The reversed field pinch magnetohydrodynamics (MHD) is characterized by a broad spectrum of interacting resistive tearing modes with poloidal mode number mainly m=0,1. According to the nonlinear coupling theory, the modes phase lock together if their amplitudes exceed a critical value. In the Reversed Field Pinch Experiment (RFX) [L. Fellin et al., Fusion Eng. Des. 25, 315 (1995)] they are always locked in phase, determining a global nonaxisymmetric magnetic perturbation. In this work a detailed experimental characterization of the magnetic perturbation is presented. The typical toroidal profiles and spectra, for both the m=0 and the m=1 components, are shown. In particular the phase relationship under locking conditions between the interacting m=0,1 modes is investigated and compared with the theoretical predictions. The results of three-dimensional (3D) MHD numerical simulations are also discussed.