Study of Low-Frequency Core-Edge Coupling in a Tokamak: I. Experimental Observation in KSTAR

TL;DR

This study investigates low-frequency fishbone events in KSTAR tokamak discharges, analyzing their dependence on plasma parameters and the relationship between edge and core plasma fluctuations.

Contribution

It provides experimental observations of fishbone behavior, decomposes relevant signals into amplitude and phase components, and explores the active role of edge activity in fishbone phenomena.

Findings

Fishbone strength increases with higher $eta_{ ext{N}}$ and lower $q_{95}$.

Edge electron temperature fluctuations become more correlated with poloidal magnetic fluctuations as fishbone strength increases.

Phase analysis suggests edge activity may actively influence core fishbone behavior.

Abstract

Double-peaked fishbone events across multiple KSTAR discharges are investigated. The normalized beta $\beta_{\mathrm{N}}$ and the edge safety factor $q_{\mathrm{95}}$ under which the fishbones appear vary depending on the presence and form of external magnetic perturbations. The fishbone strength is closely related to $\beta_{\mathrm{N}}$ and $q_{\mathrm{95}}$: as $\beta_{\mathrm{N}}$ increases and $q_{\mathrm{95}}$ decreases, the fishbone strength increases. Measured fishbone-relevant signals are decomposed into amplitude envelope and phase components in the temporal domain, which are analyzed separately. In terms of the amplitude envelope component, the edge electron temperature fluctuation $\tilde T_\mathrm{e}^{\mathrm{Edge}}$ becomes more correlated with the poloidal magnetic fluctuation $\dot{B}_\mathrm{\theta}$ compared to the core electron temperature fluctuation $\tilde T_\mathrm{e}^{\mathrm{Core}}$ as fishbone strength increases. In terms of the phase component, the phase of $\tilde T_\mathrm{e}^{\mathrm{Edge}}$ precedes the phase of $\tilde T_\mathrm{e}^{\mathrm{Core}}$ except in the case of very weak fishbones where the phase relations are inconclusive due to weak fishbone activity at the edge plasma, which is comparable to background fluctuations. The investigation suggests the possibility that the edge activity is not a mere side effect of the core activity, but could play an active role.