How Multiscale Interaction of Peeling-Ballooning Modes and Drift Waves Regulates the Pedestal and ELMs in DIII-D Wide-Pedestal Quiescent H-mode

Combining theory and modeling, we reveal that the interaction of scale-separated modes plays a key role in regulating the pedestal of wide-pedestal quiescent H-mode (WPQH) and maintaining a mixed-turbulent ELM-stable state. In the pedestal, both experimental diagnostics and numerical modeling find: a) large-scale, low-frequency peeling-ballooning mode rotating in the ion diamagnetic direction and b) small-scale, high-frequency drift wave turbulence propagating in the electron diamagnetic direction. These modes lead to a local ‘flat spot’ in the nonlinear simulation consistent with experiments^[1]. The simulation with low-k initial perturbation leads to the ELM crash predicted by the linear PB theory, while the all-scale simulation forms a "mixed turbulent" state and no ELM crash. This result shows a shifted boundary for the onset of ELMs via the effect of drift wave scattering on PBM. A reduced theoretical model is derived to explain the interplay of the observed multi-scale modes. In addition, the drift waves scatter PBM, allowing the latter to remain stable above the linear threshold for an ELM. This work sheds light on a possible fundamental mechanism of WPQH and the merit of a turbulent pedestal in future tokamak reactors.