Localized turbulence suppression and increased flow shear near the q=2 surface during internal-transport-barrier formation

Broadband turbulent fluctuations in the plasma density are transiently suppressed when low-order rational q surfaces first appear in negative central magnetic shear plasmas on the DIII-D tokamak, which can lead to the formation of internal transport barriers. Increased localized flow shear is simultaneously observed. It transiently exceeds the measured turbulence decorrelation rate, providing a mechanism to trigger the formation of the transport barrier. This increased flow shear and turbulence suppression propagates radially outward, following the q=2 surface.