Edge structure in JT-60U high density H-mode plasmas

The edge structure of the high density H mode has been investigated with emphasis on the L-H transition and degradation of improved confinement. It was thereby found that the substantial increase of the L-H threshold power at high density is related to the nonlinear increase of edge density, which significantly raises the edge collisionality. As to the quality of the edge barrier, the width of Er shear layer is reduced in high density ELMy H mode, which corroborates the decrease of the pedestal width. The interaction between the internal and edge structures has also been studied in JT-60U, where the effectiveness of the active control of internal barrier has been manifested to increase the edge density and induce the L-H transition. In addition, the role of neutral particles at the edge on L-H transition and sustainment of the edge barrier quality is addressed in this paper. (author)