Role of the edge electric field and microturbulence in the L-H transition

Two clear signatures of the L-H transition in the DIII-D tokamak are that the edge poloidal rotation velocity υθ abruptly increases, implying that the radial electric field Eτbecomesmorenegative,andthelevelofdensityfluctuationsisabruptlysuppressedinanarrowregionattheplasmaedge.TheabsolutevaluesofEτandυθhavemaximawithin1-2poloidalgyroradiiρiθoftheplasmaedge,indicatingthatlargenegativeandpositivegradientsofEτandυtheta exist near the plasma edge in a region called the ''shear'' layer. Density fluctuations are reduced and the gradients of Ti, Te, ne and the carbon density increase in the shear layer at the time of transition. These data qualitatively support theoretical scenarios in which changes in Eτ or of the gradients of Eτ or υθ cause a suppression of microturbulence, resulting in increases in the Ti and density gradients. 22 refs., 3 figs