Turbulence reduction in internal transport barriers on ASDEXUpgrade

A reduction in turbulent density fluctuations is observed in internaltransport barrier (ITB) discharges in the ASDEX Upgrade tokamak with an L-modeedge and reversed magnetic shear. The turbulence reduction is localizedspatially to around the ITB gradient and the ITB foot-point region, coincidentwith strong shearing in the toroidal rotation velocity. The radial extent anddegree of reduction is related to the strength of the ITB. Strong (narrow andsteep) ITBs are well localized and display greater turbulence reduction.Weaker ITBs are less localized with less reduction. The formation of asubsequent H-mode edge barrier moves the velocity shearing region to the edgeand weakens the ITB to the point of extinction. During the ITB the turbulenceis reduced across a broad frequency range, however a residual level oflow-frequency fluctuations (f<10 kHz) remains. Comparison of linear growthand shearing rates suggest that E×B shear may be a factor in theturbulence reduction.