MODIFICATIONS IN TURBULENCE AND EDGE ELECTRIC-FIELDS AT THE L-H TRANSITION IN THE DIII-D TOKAMAK

The L to H transition in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] is associated with two clear signatures: edge density fluctuations are abruptly suppressed (in almost-equal-to 100-mu-sec), while the edge poloidal rotation velocity upsilon-theta increases, implying that the radial electric field E(r) becomes more negative. Detailed new spectroscopic profile measurements show that the changes in upsilon-theta and E(r) generate a region of sheared electric field and poloidal flow of width almost-equal-to 3-5 cm. This region develops simultaneously with, and has the same spatial extent as, the edge fluctuation suppression zone as measured using a reflectometer system. Furthermore, the radial extent of the shear and fluctuation suppression zones encompass the location of the H-mode edge transport barrier. These observations are consistent with recent theoretical models of the L-H transition, and a comparison with these theories is presented. Data are also presented on the evolution of edge parameters and density fluctuations after the transition: the shear and fluctuation suppression layers are maintained for the duration of the quiescent H-mode phase, while relative density fluctuation levels decrease and interior plasma confinement gradually improves. Precursors to several different types of edge localized mode (ELM's) are also discussed.