Radial profiles of the poloidal velocity of fluctuations and parallel flows have been simultaneously investigated in the proximity of the last closed flux surface in the TJ-II stellarator. It has been observed that sheared perpendicular and parallel flows are linked. The parallel flow is affected by the degree of instability in the plasma edge and its radial gradient is near the threshold of Kelvin–Helmholtz instabilities. Furthermore, parallel flows and edge instabilities are dynamically coupled. These results show that, at least partially, parallel flows are controlled by plasma turbulence. Plasma profiles, parallel flows and fluctuations have also been investigated in the proximity of low order rational surfaces in the plasma boundary region. Evidence of quasi-coherent modes and a modification in the level of fluctuations have been observed near the 4/2 rational surface located in the plasma boundary region. Parallel flows show differences in configurations with and without low order rationals: a minimum appears at the outer side of the rational location, whereas in the configurations free of rationals, this minimum disappears. Plasma edge biasing is an effective tool to induce changes in parallel and perpendicular flows.
Intrinsic suppression of turbulence in linear plasma devices