The Role of Edge Resonant Magnetic Perturbations in KSTAR L-H Transitions: A Statistical and Geometric View

Understanding the low-to-high confinement mode (L-H) transition is essential for achieving H-mode access. Despite over four decades of research, key questions remain—particularly concerning the roles of hidden variables and magnetic fluctuations. Meanwhile, H-mode operation is often accompanied by edge-localized modes (ELMs), which, if uncontrolled, can cause serious damage to plasma-facing components. Although techniques such as resonant magnetic perturbations (RMPs), pellet injection, and gas puffing have shown promise in suppressing or mitigating ELMs in various tokamaks, the precise conditions required for consistent control—and the nature of RMP interactions with magnetic fluctuations—is not fully understood.

In this talk, we present recent results from the KSTAR tokamak examining how edge-localized resonant magnetic perturbations (ERMPs) influence the L-H transition. Unlike conventional RMPs, which can degrade core confinement and induce undesirable instabilities such as locked modes, ERMPs in KSTAR preferentially target edge-localized modes while preserving strong core performance. In the upper single null (USN) configuration with a carbon divertor, ERMPs are found to play multiple roles: delaying the L-H transition, suppressing ELMs and coherent edge modes, triggering the H-L back transition, inducing hysteresis, and modifying the magnetic fluctuation spectrum. Notably, ELM suppression is observed alongside an increase in Er shear. Furthermore, the impact of magnetic fluctuations varies sensitively with plasma density.To better understand these dynamics, we employ statistical techniques based on information geometry to analyze correlations among key variables across both spatial and temporal domains. This analysis provides new insights into the complex interplay between magnetic perturbations and confinement transitions.