A low-frequency (~1 kHz) oscillation (LFO) in the pedestal region emerged during the ELM-free phase after the L–H transition on the EAST tokamak. Its characteristics were studied. This oscillation has a magnetic component with a toroidal mode number (n) of 0, indicating an axisymmetric mode, while its poloidal magnetic structure demonstrates an m = 1 standing wave with in–out asymmetry. The density fluctuation shows no phase difference radially, indicating that it is not a radial traveling pulse. Before this axisymmetric oscillation, pedestal density fluctuation was dominated by low-frequency (LF) fluctuation with a frequency of less than 400 kHz. During the phase of axisymmetric oscillation, this LF fluctuation was modulated by this axisymmetric oscillation and thus experienced 'suppressed' and 'recovery' phases alternately. As a result, both pedestal density and the particle flux onto the divertor were modulated. During the 'suppressed' phase of the LF fluctuation, a new high-frequency (HF) fluctuation appeared, with a frequency larger than 500 kHz. The result demonstrates complex turbulence behavior in pedestal region.
Observation of a quasi-coherent high-frequency electromagnetic mode at the pedestal region in EAST RF-dominant H-modes