Toroidal phase locking phenomena and modal interactions with the mean fields in the Extrap T1 reversed-field pinch

Results from a study of the nonlinear dynamics of tearing modes and their interaction with the current profile in the Extrap T1 reversed-field pinch, (R/a = 0.5 m/0.057 m = 8.8), are reported. Arrays of edge coils and an insertable magnetic probe are used to study the Fourier mode structure of the magnetic field fluctuations and the associated evolution of the magnetic field profiles over the range Θ = BΘ/ = 1.6-2.4. We find that phase locking of the dominant m = 1 and m = 0 modes, due to strong nonlinear interaction, introduce toroidal asymmetries which tend to dominate the global plasma dynamics at all Θ. These m = 1/m = 0 mode lockings rotate around the torus in the electron drift direction, and give rise to large oscillations in local measurements of the toroidal flux and edge field. (author) 3 refs., 6 figs