Neoclassical tearing modes for a finite ratio of ion gyroradius to magnetic island width

A model of neoclassical tearing modes (NTMs) allowing for a finite width of the transition layer near the island separatrix, where perturbed plasma pa rameters vary significantly, is proposed. In contrast to traditional models of NTMs, this model includes the parameter rho (i)/w properly, where rho ( i) is the ion Larmor radius, and w is the magnetic island width. It is show n that both the polarization current and bootstrap current effects on these modes prove to be weakened compared with those predicted by traditional th eories of NTMs. The competition of these effects results in a critical beta for NTM onset which is lower than that usually used for interpretation of experimental observations and predictions for reactor-grade tokamaks.