Distinct stages of radio frequency emission at the onset of pedestal collapse in KSTAR H-mode plasmas

TL;DR

This study identifies distinct stages of radio frequency emission during pedestal collapse in KSTAR H-mode plasmas, revealing links to edge perturbations and ion dynamics, which could improve understanding of plasma stability.

Contribution

It introduces a high-speed RF spectrum analysis method to observe RF emission stages and their relation to edge perturbations during pedestal collapse in tokamak plasmas.

Findings

Identification of multiple RF emission stages at pedestal collapse

Correlation between RF spectra and edge perturbation structures

Observation of rapid chirping during pedestal collapse

Abstract

Using a high-speed and broadband radio frequency (RF) (0.1-1 GHz) spectrum analyzer developed on the KSTAR tokamak, it is found that several distinct stages of RF emission appear at the pedestal collapse in high confinement discharges. Comparison with 2-D electron cyclotron emission (ECE) images has revealed that each stage is related to the instantaneous condition at the outboard mid-plane edge. First, high-harmonic ion cyclotron emissions (ICE) are intensified with the appearance of a non-modal filamentary perturbation in the edge within several tens of microseconds before the collapse. Then, the RF emission becomes broad toward high-frequency range (< 500 MHz) at the burst onset of the non-modal filament. During the pedestal collapse initiated by the filament burst, rapid chirping (1-3 {\mu}s) appear with additional filament bursts. The strong correlation between the RF spectra and the perturbation structure provides important clues on the stability of edge-localized modes and on the ion dynamics in the plasma boundary.