Two-dimensional imaging of the dynamics of edge-localized filaments in KSTAR H-mode plasmas

The temporal evolution of edge-localized modes (ELMs) has been studied using a 2-D electron cyclotron emission imaging (ECEI) system in the KSTAR tokamak. The ELMs are observed to evolve in three distinctive stages: the initial linear growth of multiple filamentary structures having a net poloidal flow, the interim state of regularly spaced saturated filaments, and the final crash through a short transient phase characterized by abrupt changes in the relative amplitudes and distance among filaments. The crash phase, typically consisted of multiple bursts of a single filament, involves a complex dynamics; poloidal elongation of the bursting filament, development of a fingerlike bulge, and fast localized burst through the finger. Substantial alterations of the ELM dynamics, such as mode number, poloidal flows, and crash time scale, have been observed under external magnetic perturbations with the toroidal mode number n=1.