Experimental study of quasi-coherent mode using EAST tangential CO2 laser collective scattering diagnostic in far-forward mode

A quasi-coherent mode (QCM) was measured by the tangential CO2 laser collective scattering diagnostic at high plasma electron density during both enhanced Da/small edge-localized mode (ELM) and ELM-free H mode phases in Experimental Advanced Superconducting Tokamak (EAST). Experimental results from only local oscillator CO2 laser scattering prove that the QCM is measured by the scattering diagnostic in the far-forward mode. The driven QCM density fluctuation (k< 3 cm-1) and magnetic fluctuation suggest that the QCM is an electromagnetic mode. The typical frequency of the QCM is f≈26.5 kHz. A combination analysis of scattering signals and Mirnov signals suggests that the QCM has toroidal mode number n≈17 and rotates along with the electron diamagnetic drift velocity direction in the lab frame. The analysis of Mirnov and reflectometer signals supports thatthe QCM locates in the edge pedestal region. The QCM power has been found to be related to both the Da signal and the pedestal density gradient. A comparison of the EAST QCM and C-Mod quasi-coherent mode has been given in detail.