Experimental evidence of fluctuations and flows near marginal stability and dynamical interplay between gradients and transport in the JET plasma boundary region

The structure of the naturally occurring velocity shear layer and the dynamical coupling between gradients and transport have been investigated in the JET plasma boundary region. The velocity shear layer appears to organize 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 E×B sheared flows organized themselves to be close to marginal stability (i.e. ωE×B≈1/τ). The size of turbulent events increases when the plasma deviates from the average gradient. The resulting radial velocity of fluctuations is of the order of 20 m/s for transport events implying a small deviation from the most probable gradient. This value is consistent with a diffusive modeling with B2-Eirene. The radial velocity increases up to 500 m/s for large transport events.