The onset distribution of rotating tearing modes and its consequences on the stability of high-confinement-mode plasmas in DIII-D

Abstract

Analysis of a multi-scenario database of over 13 000 DIII-D H-mode discharges shows that the $m,n = 2,1$ magnetic islands are dominantly pressure gradient driven, stochastically triggered non-linear instabilities at all edge safety factor ( $q_\text{95}$ ) values. The instability onset time closely follows the exponential distribution in intermediate and high $q_\text{95}$ 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 $\beta_\text{N}$ flattop, constant λ throughout the $\beta_\text{N}$ flattop shows that the tearing onset is insensitive to the evolution of the equilibrium current profile. In low $q_\text{95}$ scenarios, where a large fraction of the plasmas are operated at low torque, λ increases over the course of the $\beta_\text{N}$ flattop, showing that these plasmas evolve toward more unstable conditions. The onset rate rapidly increases with $\beta_\text{N}$ , 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.

Keywords

• tearing
• statistics
• instability
• tokamak
• magnetic island