Spatial structure of low-frequency fluctuations throughout the transition of poloidal flow velocity in edge plasmas of LHD

It is observed that a low-frequency (∼2 kHz) density fluctuation is excited in a transition of the poloidal flow velocity (V_θ) in the edge magnetic stochastic region of the Large Helical Device plasmas. Furthermore, it is found that the propagation velocity becomes approximately zero in the proximity of the appearance region of the low-frequency fluctuation by using the edge multichannel microwave Doppler reflectometer system. In particular, the low-frequency fluctuation is considered to be transmitted in both directions (inward and outward) away from the excitation position of the fluctuation, which behaves as the precursor of a magnetic fluctuation burst. Afterward, the edge H α signal intensity is sharply increased.