New results for the finite ion orbit width effect on NTM threshold

For successful operation of future tokamaks, such as ITER, it is essential to control neoclassical tearing modes (NTMs): plasma instabilities characterized by the evolution of magnetic islands. Experiments show that sufficiently small magnetic islands heal themselves and shrink away. Possible origins of this threshold have been suggested, e.g. the effects of finite radial transport, finite Larmor radius and finite banana width. However, an NTM control system requires a deeper understanding of the threshold physics, for predicting the threshold island width w_c. This paper investigates the effect of finite particle orbit width on the NTM threshold in toroidal geometry, considering a small island width w comparable to the ion poloidal Larmor radius ρ_θi. We find that the perturbed ion distribution function profile shifts radially by O(ρ_θi), relative to the magnetic island location. When w ~ ρ_θi, this shift substantially restores the density gradient that is flattened across the island, weakening the bootstrap current drive for the island growth. The result is the threshold behavior, with the critical island width scaling as: w_c ~ 2.7ρ_θi, in the absence of other effects. These results will provide valuable information toward the development of effective NTM control system for ITER.