Scrape-off layer power fall-off length broadening in small ELM regimes on DIII-D

A DIII-D database study investigated divertor heat flux and its dependencies on pedestal parameters across a range of small ELM regimes: high beta poloidal, type-II, and ELMs in reduced negative triangularity H-modes [1]. It demonstrated that type-II and high beta poloidal small ELM regimes have their parallel energy fluences fall below the Eich type-I scaling from [2] and that the SOL power fall-off lengths were larger than predicted using the Eich scaling with poloidal magnetic field in [3]. Although on a whole the SOL power fall-off lengths were broadened, there were some stationary periods that had their power fall-off lengths follow the Eich scaling. Within each small ELM regime the stationary periods with the largest difference in power fall-off lengths were examined for differences in pedestal characteristics, radial electric fields, and edge fluctuation measurements. Recent work in [4], indicates the importance of E x B flow shear in causing changes in turbulence spreading that alters the broadening/narrowing of the power fall-off length. Understanding the commonalities of the broadened power fall-off lengths in the small ELM regimes informs feasibility of the small ELM regimes for effective core-edge integration in future fusion-grade tokamaks.

[1] P. Traverso et al. 2024 Nucl. Fusion (submitted)

[2] T. Eich et al. 2017 Nuclear Materials and Energy 12 84–90

[3] T. Eich et al 2013 Nucl. Fusion 53 093031

[4] N. Li et al 2024 J. Plasma Phys 90 905900117