Understanding the roots of tearing mode onset and growth in DIII-D

A new database study of >2600 DIII-D H-mode discharges assesses the dominant drive of 2,1 tearing modes (TMs), their parameter sensitivities, and the relative occurrence of different seeding mechanisms. As TMs degrade confinement and can lead to disruptions, resolving TM onset physics is crucial for projecting stabilization and avoidance strategies to ITER and beyond. Across the analyzed dataset, 2,1 amplitude grows linearly in the majority of shots, implying that neoclassical stability dominates the mode evolution. The observed onset sensitivities to β_N, q_min, and shape parameters also show qualitative agreement with neoclassical TM theory. The onset time distribution follows Poisson point statistics, which is consistent with a random trigger plus a threshold for robust growth. The relative importance of various seed mechanisms such as sawteeth, ELMs, and 3-mode coupling is quantified across reactor-relevant scenarios, and suggests different scenario-dependent approaches to avoiding deleterious 2,1 activity. The results of this work can also help inform the development of scenarios that are passively stable to disruptive 2,1 islands. Work supported by US DOE under DE-FC02-04ER54698.