Experimental evidence of fluctuations and flows near marginal stability in the plasma boundary region of fusion plasmas

Electrostatic turbulence has been investigated in the plasma boundary region in the JET tokamak and in the TJ-II stellarator. In both devices the naturally occurring velocity shear layer organizes itself to reach a condition in which the radial gradient in the poloidal phase velocity of fluctuations is comparable to the inverse of the correlation time of fluctuations (1/τ). This result suggests that ExB sheared flows organized themselves to be close to marginal stability (i.e. ωExB ≈ 1/τ). The investigation of the dynamical interplay between fluctuation in gradients, turbulent transport and radial electric fields has shown that these parameters are strongly coupled both in tokamak and stellarator plasmas. The bursty behaviour of turbulent transport is linked with a departure from the most probable radial gradient. The dynamical relation between fluctuations in gradients and transport is strongly affected by the presence of sheared poloidal flows, heating power and the proximity to instability thresholds: the size of large transport events decreases in the proximity of sheared flows and increases with heating power and in the proximity of instability thresholds.These results are consistent with the concept of turbulent transport self-regulated via fluctuations near marginal stability.