Direct measurement of poloidal long-wavelength [formula presented] flows in the ht-7 tokamak

The poloidal long-wavelength [Formula presented] time-varying flows were directly measured using a forked Langmuir probe in the HT-7 tokamak. Low-frequency ([Formula Presented]) [Formula presented] flows were observed at the plasma edge, which possess many of the characteristics of zonal flows, including a poloidal long-wavelength ([Formula presented]) and narrow radial extent ([Formula presented]). The cross bicoherence of turbulent Reynolds stress indicates the existence of nonlinear three-wave coupling processes and the generation of low-frequency [Formula presented] flows. The estimated flow-shearing rate is of the same order of magnitude as the turbulence decorrelation rate and may thus regulate the fluctuation level and thereby the turbulence-driven transport.