Mode properties over Quantity: Revisiting GAE-Driven Transport Thresholds

Previous studies by Gorelenkov et al. [1] suggested that substantial electron thermal transport in spherical tokamaks requires interaction with at least 15 Global Alfvén Eigenmode (GAE) fluctuations. In this work, we re-examine that conclusion using guiding-center simulations with the same set of 30 theoretically and experimentally motivated modes, with the same experimentally motivated amplitude, α₀ = 4x10^-4 where α₀= A_∥/R₀B₀ is the normalized parallel vector potential. Surprisingly, we find that strong anomalous electron transport can be generated by just three well-chosen modes—referred to here as “triplet modes”—highlighting the critical role of mode properties over mode count. We identify three distinct triplets that consistently drive robust, phase-insensitive transport. Additionally, we discover three “transport-enhancing” modes that do not induce transport alone but significantly amplify it when combined with ensembles of eight or more modes. Conversely, we identify 21 “neutral” modes that produce negligible transport—even when exceeding the 15-mode threshold—unless at least one transport-enhancing mode is included. These findings indicate that previously reported mode thresholds emerged from the inclusion of transport-enhancing modes. Lastly, we show that triplet and neutral-mode-driven transport is largely independent of the initial phase, in contrast to large-mode ensembles where phase plays a critical role.

Ref. :

[1] Gorelenkov et al., Nucl. Fusion 50 084012 (2010).