Turbulence reduction in internal transport barriers on ASDEX Upgrade

A reduction in turbulent density fluctuations is observed in internal trans port barrier (ITB) discharges in the ASDEX Upgrade tokamak with an L-mode e dge and reversed magnetic shear. The turbulence reduction is localized spat ially to around the ITB gradient and the ITB foot-point region, coincident with strong shearing in the toroidal rotation velocity. The radial extent a nd degree of reduction is related to the strength of the ITB. Strong (narro w and steep) ITBs are well localized and display greater turbulence reducti on. Weaker ITBs are less localized with less reduction. The formation of a subsequent H-mode edge barrier moves the velocity shearing region to the ed ge and weakens the ITB to the point of extinction. During the ITB the turbu lence is reduced across a broad frequency range, however a residual level o f low-frequency fluctuations (f < 10 kHz) remains. Comparison of linear gro wth and shearing rates suggest that E x B shear may be a factor in the turb ulence reduction.