Edge plasma parameters at the L-H transition under the conditions of open and W-shaped divertor in JT-60U

A substantial reduction of the L-H transition thresholdpower was observed under the W-shaped divertor in JT-60U, in comparisonwith the open divertor. Radiation power in the main region was alsodecreased in this case. These differences of radiation power from themain plasma between open and W-shaped divertor were not large enough toaccount for the apparent reduction of threshold power. In this paper,the emphasis is on the edge plasma parameters which relate to the L-H transition. It was found that edge plasma pressure just before the L-Htransition in the W-shaped divertor case became smaller than that in theopen divertor case. After the modification of divertor geometry, edgeion collisionality just before the L-H transition also became lower andit was established with lower auxiliary power input. It was suggestedthat reduction of edge ion collisionality arose from neutral particlesnear the X-point by the analysis of poloidal profile of neutral density.Therefore, neutral particles near the X-point prevented the thresholdpower for the L-H transition from more reduction at low plasma density,and caused the shift of plasma density which gave minimum thresholdpower.