Fast RF diagnostics in KSTAR ELMy H-mode discharge

A fast radio frequency (RF) (0.1 - 1 GHz) diagnostics has been developed in KSTAR. Using fast digitization technology (∼ 5 GSa/s), it becomes possible to measure the dynamic changes of the RF emission with sub-μs time resolution. Especially, we found that there exist several distinct stages of RF emission at the pedestal collapse in H-mode discharges. Comparison with the electron cyclotron emission (ECE) images has revealed that each stage is related to the condition of the outboard mid-plane edge. First, intensified high-harmonics ion cyclotron emissions (ICE) occur with the appearance of a non-modal filamentary perturbation in the edge within several tens of μs before the collapse. Then, the RF signal becomes broad toward high-frequency range (< 500 MHz) at the crash onset of the non-modal filament. During the collapse, rapid frequency up/down chirp signals (1-3 μs) appear with additional filament bursts. These RF observations can provide clues for ion dynamics at the pedestal collapse. (author)