Observation of a quasi-coherent high-frequency electromagnetic mode at the pedestal region in EAST RF-dominant H-modes

A quasi-coherent high-frequency mode (HFM), with frequency similar to 300 kHz, has been observed in EAST H-modes obtained by lower hybrid current drive or combined heating of lower hybrid wave and ion cyclotron resonance frequency wave after lithium wall conditioning. The HFM with an initial growing phase in frequency and amplitude appears both following L-H transition and between edge-localized modes at a relatively high-collisionality.v(e)(*). Detailed analysis from edge Langmuir probe has revealed that the HFM propagates in the electron diamagnetic direction in both the lab frame and the plasma frame. The decorrelation rate ofHFMis smaller than the maximumExB shearing rate at the steep gradient region by an order of magnitude. Furthermore, the growth, saturation and oscillations of HFM, including both frequency and amplitude, are strongly correlated with the evolution of pressure gradient. Thus, the limit-cycle-like oscillations between the amplitude and the frequency of HFM may play an important role in the formation and saturation of pedestal. During the saturation phase of pedestal pressure gradient, the HFM, sometimes accompanied by a mid-frequency (similar to 80 kHz) quasi-coherent mode, drives strong cross-field transport resulting in small crashes of pressure gradient and finally limits the pressure gradient.