Phase-locking of tearing modes in the reversed field experiment

In the reversed field experiment ~RFX! @F. Gnesotto et al., Fusion Eng. Des. 25, 335 ~1995!#, the m51 and m50 tearing modes present in the plasma are observed to phase-lock together to form a highly peaked, strongly toroidally localized, pattern in the perturbed magnetic field. This pattern, which is commonly known as the ‘‘slinky’’ pattern, gives rise to severe edge loading problems which limit the maximum achievable toroidal current. A theory is presented which explains virtually all salient features of the RFX slinky pattern. The central premise of this theory is that at high ambient mode amplitude the various tearing modes occurring in the plasma phase-lock together in a configuration which minimizes the magnitudes of the electromagnetic torques exerted at the various mode rational surfaces. The theory successfully predicts the profiles of the edge radial and toroidal magnetic fields generated by the m50 and m51 modes, the phase relations between the various modes, the presence of a small toroidal offset between the peaks of the m50 and m51 contributions to the overall slinky pattern, and the response of the pattern to externally generated m50 and m51 magnetic perturbations. © 2002 American Institute of Physics. @DOI: 10.1063/1.1481057#