Analysis of a multi-scenario database of over 13 000 DIII-D H-mode discharges shows that the magnetic islands are dominantly pressure gradient driven, stochastically triggered non-linear instabilities at all edge safety factor () values. The instability onset time closely follows the exponential distribution in intermediate and high scenarios and is characterized by near constant onset rate (λ), in accordance with Poisson-point processes. This implies that the plasmas are operated in marginally stable conditions, characterized by a small threshold for instability growth and variations in the trigger amplitude and/or the stabilizing mechanisms with temporally uniform random distribution in this database. While the majority of the tearing modes occur in the first current-profile relaxation time of the flattop, constant λ throughout the flattop shows that the tearing onset is insensitive to the evolution of the equilibrium current profile. In low scenarios, where a large fraction of the plasmas are operated at low torque, λ increases over the course of the flattop, showing that these plasmas evolve toward more unstable conditions. The onset rate rapidly increases with , while it does not show a clear dependence on the current gradient at the mode rational surface. Overall, these observations support that the majority of the analyzed 2,1 tearing modes are non-linear, neoclassically driven instabilities and classical stability does not play a dominant role in their onset.
Nonlinear evolution of tearing modes in inhomogeneous plasmas