In the present study, fluctuation characteristics have been investigated using beam emission spectroscopy in discharges with the edge transport barrier in the compact helical system. If the heating power was high enough, a coherent mode having fundamental (f = 4–5 kHz) and 2nd harmonic (2f = 8–10 kHz) frequencies was observed. This mode showed similar characteristics to those of the edge harmonic oscillations (EHO) observed in tokamaks. This EHO-like mode is located in the edge region where steep density gradients and the ι = 1 rational surface exist. There seems to be a threshold in the density gradient for enhancing the EHO-like mode. After the excitation of the EHO-like mode, the density gradient saturates. The edge density gradient remained constant when the EHO-like mode existed, even if the heating power was increased, suggesting that there is some correlation between the EHO-like mode and particle confinement.
Experimental identification of I-mode characteristics at the edge of FIRE mode in KSTAR