Edge-localized modes on KSTAR: Global structure and distinct evolution stage involving quasi-steady state and pahse transitions

Abstract

Edge-localized modes (ELMs) in the KSTAR tokamak appear substantially different from the conventional picture of ELMs as an explosive transport event in the plasma edge triggered by exponentially growing ballooning and external kink modes on the low field side of the plasma. The 2D images of ELMs visualized by an advanced imaging diagnostics r1s with microsecond time resolution revealed that the modes evolve in three distinctive stages: 1) quasi-steady (saturated) filamentary mode r2s with long life time (up to „100 ms), 2) abrupt structural transformation near the onset of crash into irregularshaped filaments r2s or in the middle of the intercrash period r3s, and 3) multiple filament bursts during the crash phase r2s. Perhaps the most astonishing finding is the clear existence of filamentary modes exist on the high field side as well as on the low field side r4s, suggesting that the ELM dynamics in the KSTAR involve other driving forces such as d’Angelo instability besides ballooning and external kink. In addition, we demonstrate that the ELM evolution stages are associated with distinct changes of RF emission spectra r5s (100–1000 MHz), suggesting the RF signal as a better alternative to the conventional Dα signal which represents only the aftermath of the collapse of the edge confinement.