Study of low-frequency core-edge coupling in a tokamak: I. Experimental observation in KSTAR

Double-peaked fishbone events across multiple KSTAR discharges are investigated. The normalized beta βN and the edge safety factor q₉₅ under which the fishbones appear vary depending on the presence and form of external magnetic perturbations. The fishbone strength is closely related to βN and q₉₅: as βN increases and q₉₅ 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 TₑEdge becomes more correlated with the poloidal magnetic fluctuation Ḃ_θ compared to the core electron temperature fluctuation TₑCore as fishbone strength increases. In terms of the phase component, the phase of TₑEdge precedes the phase of TₑCore 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.