Evidence for modified transport due to sheared ExB flows in high-temperature plasmas

Sheared mass flows are generated in many fluids and are often important for the dynamics of instabilities in these fluids. Similarly, large values of the ExB velocity have been observed in magnetic confinement machines and there is theoretical and experimental evidence that sufficiently large shear in this velocity may stabilize important instabilities. Two examples of this phenomenon have been observed in the DIII endash D tokamak. In the first example, sufficient heating power can lead to the L-H transition (transition from low-mode to high-mode confinement), a rapid improvement in confinement in the boundary layer of the plasma. For discharges with heating power close to the threshold required to get the transition, changes in the edge radial electric field are observed to occur prior to the transition itself. In the second example, certain classes of discharges with toroidal momentum input from neutral beam injection exhibit a further improvement of confinement in the plasma core leading to a regime called the VH endash mode. In both examples, the region of improved confinement is characterized by an increase of shear in the radial electric field Er, reduced levels of turbulence and increases in gradients of temperatures and densities. These observations are consistent with the hypothesis that the improved confinement is caused by an increase in shear of the ExB velocity which leads to a reduction of turbulence. For the VH endash mode, the dominant term controlling Er is the toroidal rotation vφ, indicating that the Er profile is controlled by the source and transport of toroidal momentum. At the edge of the plasma, indirect measurements indicate that the change in Er is initiated by a change in the VxB velocity of the main ions but at later times is dominated by the ion diamagnetic velocity. copyright 1995 American Institute of Physics