Description
The H-mode in tokamaks brings fusion performance within reach but faces two major challenges: damaging ELMs and excessive divertor heat loads. Recent results across tokamak experiments identified an ELM-free H-mode—the quasi-continuous exhaust (QCE) regime—combining high pedestal pressure, reactor-relevant separatrix density, and a broadened the scrape-off layer (SOL) heat width, making it promising for a fusion reactor [1]. A first-principles understanding of the turbulence dynamics in QCE plasmas has been elusive, given the complexities of nonlinear, separatrix-spanning electromagnetic modes and their interactions with large-amplitude, intermittent filaments in the SOL.
We address the challenge using two-fluid turbulence code GRILLIX [2,3]. We conduct global simulations of the QCE regime across the pedestal and SOL in realistic ASDEX Upgrade geometry. The results reveal a quasi-coherent mode (QCM) [4] spanning the separatrix and launching ballistic, filamentary blobs into the SOL, reproducing not only mean profiles but also fluctuation spectra and mode structure seen in experiments.
We identify QCM as a kinetic ballooning mode (KBM) with a long radial correlation length, which yields transport levels exceeding those of a KBM-unstable ELMy pedestal foot. The transport by QCM in the pedestal foot is regulated by two key ingredients: the self-consistent formation of a broad Er well, and the finite Larmor radius physics. The resistivity remains prominent near the X-point and triggers a secondary resistive X-point mode (RXM) [5]. The emergence of RXM strengthens interchange dynamics at the outermost extent of the QCM, thereby launching blobs into the SOL. The blob-dominated SOL temperature fall-off is then well decoupled from the pedestal-foot gradient set by the QCM. Together, these dynamics outline a pathway to self-sustained H-mode confinement with manageable heat exhaust.
[1] M. Faitsch et al., Nucl. Fusion 63, 076013 (2023).
[2] K. Zhang et al., Nucl. Fusion 64, 036016 (2024).
[3] K. Zhang et al., Computer Physics Communications 314, 109670 (2025).
[4] J. Kalis et al., Nucl. Fusion 64, 016038 (2024).
[5] J. R. Myra et al., Phys. Plasmas 7, 4622 (2000).