Observation of Doppler shift f_D modulated by internal kink mode using conventional reflectometry on EAST tokamak

In this paper, a new experimental observation will be presented by using a conventional reflectometry, called poloidal correlation reflectometry (PCR) on experimental advanced superconducting tokamak (EAST). The turbulence spectrum detected by the PCR system exhibits an asymmetry and induced Doppler shift f_D during internal kink mode (IKM) rotation phase. This Doppler shift f_D is the target measurement of Doppler reflectometry, but captured by conventional reflectometry. Results show that the Doppler shift f_D is modulated by the periodic changes in the effective angle between the probing wave and cutoff layer normal, but not by plasma turbulence. Fishbone mode and saturated long-live mode (LLM) are typical IKMs, and this modulation phenomenon is observed in both cases. Moreover, the value of Doppler shift f_D is positively correlated with the amplitude of IKM, even when the latter is small. However, the positive and negative frequency components of the Doppler shift f_D can be asymmetric, which is related to the plasma configuration. A simulated analysis is performed by ray-tracing to verify these observations. These results establish a clear link between the Doppler shift f_D and IKM rotation, and are beneficial to study the characteristics of IKM and related physical phenomena.