TOROIDAL PHASE-LOCKING PHENOMENA AND NONAXISYMMETRIC FLUX PERTURBATIONS IN THE EXTRAP-T1 REVERSED-FIELD PINCH

The structure of the magnetic fluctuations in the Extrap TI reversed-field pinch [Phys. Scr. 44, 358 (1991)] has been studied over the pinch parameter range 1.6<THETA = B(theta)(a)/[B(phi)]<2.2. It is found that nonaxisymmetric perturbations of the toroidal flux and edge field dominate the global dynamics at all THETA. 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 3.3x10(4) 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.