Physics of the L to H transition in DIII-D

A decrease in the edge magnetic and density fluctuations is always seen at the L-H transitions in DIII-D. Although H-mode is best known for an increase in energy confinement, in fact, the reduction in the edge fluctuations is one of the most reliable indicators of the L-H transition. In DIII-D, a sudden change of the perpendicular rotation, V perpendicular in the edge has been also observed at the transition. Recently, theories that fluctuations are reduced by sheared poloidal flow have been proposed to link these two observations. Also, a theory to explain the mechanism responsible for the observed poloidal flow has been proposed. Experimental observations have thus far been consistent with these theories. 6 refs., 4 figs