Rotation and improved confinement modes

The hypothesis that fluctuation levels and anomalous transport in tokamaks are suppressed by radial shear in E x B rotation is reviewed, with emphasis on the L- to H-mode transition. At present there is no unambiguous experimental support for the hypothesis and further work with better space and time resolution is required. Mechanisms which rely on ion loss at the plasma edge to provide a radial electric field appear unlikely. (Author)