Observation Of Edge Magnetic Islands And 3D Turbulence Structure During Rmp ELM Suppression

For the first time, a "phase flip" is observed in the ion temperature signal associated with a rotating n=1 edge magnetic island induced by RMP on the KSTAR tokamak, under an optimized spectral configuration [1] that enables ELM suppression without core locking. This observed phase flip serves as direct evidence of small magnetic islands forming near the pedestal top—a key region of the plasma edge—during ELM suppression. Notably, unlike in a previous study using higher n=2 RMP [2], no subtraction of the edge kink response was required to reveal this signature. The experiments also found that the phase flip, an indicator of a magnetic island, is clear only during the ELM suppression phase but not during the ELM mitigation phase, where multiple discharges are investigated to clarify potential uncertainties. In addition, a sharp drop in pedestal carbon ion temperature was observed during the transition from ELM mitigation to complete suppression, supporting the idea that edge magnetic island penetration plays an important role in ELM suppression. The study also uncovered a unique turbulent structure associated with these edge magnetic islands. Unlike the turbulence with larger core islands due to tearing modes, the turbulence near the edge islands showed distinct asymmetry relative to the X-point. These results emphasize how turbulence dynamics at the plasma edge differ fundamentally from those in the core, potentially with its smaller size than large core islands. These results provide valuable insights into the role of edge magnetic islands in ELM suppression and self-organized turbulence structure, advancing our understanding of plasma stability and the mechanisms that govern confinement in fusion devices.