Toroidal phase locking phenomena and nonaxisymmetric flux perturbations in the Extrap T1 reversed-field pinch

The structure of the magnetic fluctuations in the Extrap T1 reversed-field pinch [Phys. Scr. 44, 358 (1991)] has been studied over the pinch parameter range 1.6<Θ=Bθ(a)/〈Bφ〉<2.2. It is found that nonaxisymmetric perturbations of the toroidal flux and edge field dominate the global dynamics at all Θ. These perturbations are formed by phase locking of the dominant m=1 modes and m=0 modes due to strong nonlinear interaction. This m=1/m=0 mode locking rotates toroidally in the electron drift direction, with an angular rotation velocity of typically 3.3×104 rad/s. The toroidal mode numbers of the m=0 modes are consistent with being nonlinearly driven by the m=1 modes. Evidence is found that the behavior of the leading m=1 mode governs the degree of phase locking. The role of this phenomenon in mean field maintenance is discussed.