Dynamics of the pedestal in the recovery phase in EAST type-I ELM plasmas

In the Experimental Advanced Superconducting Tokamak high-confinement mode plasmas, a low-frequency n = 1 magnetic coherent mode with frequency f = 20-50 kHz and a high-frequency mode with electromagnetic characteristics (HFEM, f similar to 280 kHz) have been found between type-I edge localized modes (ELMs). Both the HFEM and the n = 1 mode are located in the pedestal region, but their radial locations seem to be somewhat different. It seems from the present data that the HFEM is closer to the maximum density gradient region, while the n = 1 mode may be closer to the separatrix. The experimental results demonstrate that the electron temperature recovers more rapidly than the pedestal density, and the n = 1 mode is excited in the pedestal after an ELM collapse. With the increase in the pedestal density, the HFEM appears and becomes dominant, while the amplitude of the n = 1 mode decreases significantly. The observations indicate that the HFEM may suppress the amplitude of the n =1 mode. In the pre-ELM phase, the pedestal electron density and temperature are saturated, the characteristics of the HFEM show a significant change (a much broader frequency spectrum and reduced mode amplitude), and the n = 1 mode recovers again. Analysis using a wavelet bispectrum reveals that a nonlinear coupling between the n = 1 mode and the high-frequency magnetic fluctuations exists in the pre-ELM phase. The relations between the nonlinear mode coupling, the reappearance of the n = 1 mode and the ELM crash are discussed.