The radial wavenumber spectra of electrostatic fluctuations measured by Langmuir probe arrays on the KT-5C tokamak show a quasimode-like structure which results in a net radial outward propagation of the turbulent fluctuations. The power-averaged radial wavenumber is approximately constant in the core region and increases near the limiter. The radial wavenumber spectral width stays nearly constant from core to edge and is about two times greater than the poloidal wavenumber spectral width. The measured fluctuation levels and wave action fluxes are in good agreement with model predictions (Mattor N and Diamond P H 1994 Phys. Plasmas 1 4002, Mattor N 1995 Phys. Plasmas 2 766) which suggest that drift wave propagation could be a source of edge turbulence.

On the CT-6B tokamak, a transition from low to high plasma current discharge was obtained by feedback control of plasma current. Increased parallel flow shear (V ) was observed to be possibly correlated with enhanced fluctuation level and fluctuation-driven particle fluxes, and a more isotropic feature of edge turbulence in the perpendicular plane. A maximum of V was found to be roughly located at where the poloidal velocity shear layer was located. The results suggest that V instability may play an important role in edge turbulence.

Langmuir probes and a Mach probe were used for measuring edge fluctuations and parallel plasma flow, respectively, on the CT-6B tokamak. A maximum radial gradient in parallel flow was observed to be located roughly at the maximum Er shear layer (or, namely, the poloidal velocity shear layer). Er shear is found to be very possibly consistent with a regulating effect on edge turbulence features. Non-linear analysis indicates that non-linear phase coupling in turbulence may be influenced by Er shear, or that there exist coherent structures induced by strong Er shear, and the phase coupling processes show an intermittent character. The results provide some new observations for a better understanding of the basic mechanisms of edge turbulence.

On the China Tokamak (CT-6B) [Nucl. Fusion 36, 1669 (1996)], application of negative limiter bias resulted in enhanced Er shear in the naturally occurring Er shear layer near the fixed limiter radius with negligible change of plasma density, electron temperature, the parallel plasma flow, and the impurity ion radiation power. In the layer, decreased turbulent fluctuation, reduced poloidal correlation and increased nonlinear coupling of the turbulence were observed to be very possibly correlated with enhanced Er shear. The results suggest that there exists interaction of Er shear with turbulence, and an Er shear-induced shift in the phase angle between density and poloidal electric field fluctuations and nonlinear three-wave coupling may play an important role in suppressing edge turbulence.