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

Abstract

Double-peaked fishbone events across multiple KSTAR discharges are investigated. The normalized beta $β_{\mathrm{N}}$ 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 $β_{\mathrm{N}}$ and $q_{\mathrm{95}}$: as $β_{\mathrm{N}}$ 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 $\dot{B}_\mathrmθ$ 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.

Figures (16)

• Figure 1: (a) Histograms of double-peaked fishbone strength in KSTAR. 40 KSTAR discharges and around 3,000 fishbones are examined. Blue and green bars are distributions of the fishbone strengths found in H-mode plasmas with magnetic perturbations that cause energy confinement degradation (blue) and enhancement (green). Red bars show the distribution of the fishbone strengths in H-mode plasmas without external magnetic perturbations. (b) The edge safety factor at the 95% normalized flux surface $q_{\mathrm{95}}$ versus the normalized beta $\beta_{\mathrm{N}}$ when the fishbones are observed. Red, blue, and green dots correspond to the same color code as in (a). (c) Same as in (b) except the ordinate is $\beta_{\mathrm{N}}$ divided by the total NBI power $P_\mathrm{NBI, tot}$.
• Figure 2: Examples of the conditional averages of normalized amplitude envelopes $\left< A^\textrm{Env}/A^\textrm{Env}_\textrm{max} \right>$ (solid lines) of fishbone-relevant $\tilde{T}_\mathrm{e}$ measured by two ECE channels, i.e., $r/a=-0.05$ (blue) and $r/a=0.94$ (red). The dashed lines are conditional averages smoothed by the Savitzky-Golay filter.
• Figure 3: (a) Histograms of the double-peaked fishbones categorized into four groups depending on their strengths, and (b) their corresponding $\beta_{\mathrm{N}}$ and $q_{\mathrm{95}}$ values. Gray color indicates all the fishbones investigated in this work. Purple, green, red and blue indicate very weak, weak, moderate-strength and strong fishbones, respectively.
• Figure 4: ECE image of a double-peaked fishbone observed in KSTAR shot #26305 (weak fishbones). The upper panel shows the magnetic perturbations, with the red bar indicating the time at which the ECE image is displayed in the lower panel where the dashed lines represent the flux surfaces, and the black thick solid line is the last closed flux surface.
• Figure 5: Same as Figure \ref{['fig:26305_ECEI']} in KSTAR shot #22472 (moderate-strength fishbones).