The non-linear evolution of low mode number tearing modes is analysed, emphasizing the need for a threshold condition, to account for observations in tokamaks. The discussion is illustrated by two models recently introduced in the literature. The models can be compared with the available data and/or serve as a basis for planning experiments in order either to test theory (by means of beta -limit scaling laws, as proposed in this paper) or to attempt to control undesirable tearing modes. Introducing a threshold condition in the tearing mode stability analysis is found to reveal some bifurcation points and thus domains of intrinsic stability in the island dynamics operational space
MHD limit cycles on FTU