# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1803.09376