Nature of Edge Harmonic Oscillation (EHO) and its Dynamics

The spatial structure and temporal dynamics of edge harmonic oscillation (EHO) have been investigated using 2-D imaging diagnostics (ECEI, BES) in KSTAR ELM-free H-mode plasmas. During the presence of low-n (n≤4) EHO, edge-localized mode (ELM) filaments remained stable and the ELM crashes hardly occurred. Spectral and bispectral analysis using the imaging data show that there is a strong nonlinear interaction between EHOs, and the nonlinear interaction of EHOs has a significant effect on the ELM structure and dynamics. The EHO is highly related to the perpendicular rotation and its shear, especially the EHO appeared when the rotation shear is large compared to typical H-mode plasmas. Observed EHO appeared discontinuously and synchronized with quasi-periodic RF spikes, reducing the electron density of pedestal. These particle transport events, seen through the quasi-periodic RF spikes, seem to reduce the pressure gradient below the crash threshold and thereby play an important role in maintaining the ELM-free H-mode plasmas.