Coherent modes and turbulences observations with multi-channel Doppler reflectometer on experimental advanced superconducting tokamak

Abstract

With a newly installed 5-channel W-band Doppler reflectometer (DR) in EAST tokamak, many interesting coherence modes and turbulences have been observed. In this paper, the tearing mode (TM), a novel inter-edge localized mode (ELM) electromagnetic mode, and the eigenmode geodesic acoustic mode (GAM) accompanied by a broadband quasi-coherent mode (QCM) during H-mode stage have been reported. The TM could be directly observed in both density fluctuation and perpendicular velocity fluctuation at the DR channel localized in the magnetic island region; moreover, many high-order TM frequency harmonics, dual turbulence peaks inside and outside the island separatrix, and strong flow shear across the island separatrix could also be observed. A high-frequency (∼230 kHz) electromagnetic mode (toroidal mode number n = 1) could be identified. The mode is generated dozens of milliseconds before the ELM crash and may affect the ELM dynamic process. The eigenmode GAM excited during the H-mode stage is generated near the top of the density pedestal and then propagates radially inward with a radial wavenumber of about 1 cm−1. A broadband QCM with a frequency range of 20–100 kHz is always accompanied by eigenmode GAM and nonlinearly coupled with both GAM and the background turbulence from the bispectrum analysis. All these results convince the validity of the Doppler reflectometer in multi-scale measurement from the mesoscale to microscale.