The poloidal long-wavelength $E\times B$ time-varying flows were directly measured using a forked Langmuir probe in the HT-7 tokamak. Low-frequency ($<10\mathrm{k}\mathrm{H}\mathrm{z}$) $E\times B$ flows were observed at the plasma edge, which possess many of the characteristics of zonal flows, including a poloidal long-wavelength ($k_{\theta }{\rho }_i\sim 0$) and narrow radial extent ($k_r{\rho }_i\sim 0.1$). The cross bicoherence of turbulent Reynolds stress indicates the existence of nonlinear three-wave coupling processes and the generation of low-frequency $E\times B$ 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.

• Received 24 January 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.125001