Edge-localized modes in KSTAR: distint evolution stages and structure

Abstract

Three general stages of the edge-localized mode (ELM) evolution in the KSTAR tokamak have been identified using an advanced 2D imaging diagnostics1 with microsecond time resolution instead of relying on the conventional D-alpha signal which represents only the aftermath of the crash: (1) quasi-steady (saturated) filamentary mode2 with long life time (up to ~100 ms), (2) abrupt structural transformation (either near the onset of crash2 or in the middle of the inter-crash period3) into irregular-shaped filaments, (3) and multiple filament bursts during the crash phase2. In the quasi-steady state, it is also clearly observed that the filamentary modes exist on the inboard edge as well as on the outboard edge4 , demonstrating that the ELM dynamics in the KSTAR involve other driving forces besides ballooning and peeling. In addition, we show that the ELM evolution stages are associated with distinct RF emission spectra5 (100—1000 MHz), suggesting the RF signal as a better alternative to Dalpha signal for the study of ELMs.