Observation of Doppler shift \mathitf_\mathbfD modulated by the internal kink mode using conventional reflectometry in the EAST tokamak

In this paper we present a new experimental observation using a conventional reflectometry technique, poloidal correlation reflectometry (PCR), in the Experimental Advanced Superconducting Tokamak (EAST). The turbulence spectrum detected by the PCR system exhibits an asymmetry and induced Doppler shift f_\mathrmD during the internal kink mode (IKM) rotation phase. This Doppler shift f_\mathrmD is the target measurement of Doppler reflectometry, but captured by conventional reflectometry. Results show that the Doppler shift f_\mathrmD is modulated by the periodic changes in the effective angle between the probing wave and cutoff layer normal, but not by plasma turbulence. The fishbone mode and saturated long-lived mode are typical IKMs, and this modulation phenomenon is observed in both cases. Moreover, the value of the Doppler shift f_\mathrmD is positively correlated with the amplitude of the IKM, even when the latter is small. However, the positive and negative frequency components of the Doppler shift f_\mathrmD 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 f_\mathrmD and IKM rotation, and are helpful for studying the characteristics of IKM and related physical phenomena.