Bifurcation of generic metastable tearing modes interacting with resonant magnetic fields

The response of generic metastable tearing modes (of which neoclassical tearing modes-NTMs-are a particular example) to resonant external magnetic fields, in rotating tokamak fusion plasmas, is analyzed. In particular, the problem of the bifurcation process that may arise during the driven destabilization of the mode by the external fields is addressed. The results presented provide a clear picture of both the mode amplitude and frequency dynamics in a scenario where there is a differential rotation between the rotating plasma and the external fields. It is shown that the mode frequency exhibits a highly oscillatory behavior during the bifurcation process (owing to the differential rotation) and not a smooth transition between the driving frequency and natural mode rotation. On such grounds, the interpretation of experimental results where magnetic coupling is thought to play a role on the destabilization of some NTMs is revised. (C) 2004 American Institute of Physics.