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Abstract. Substantial reduction of the H-mode transition threshold power was observed under the W-shaped divertor in JT-60U, in comparison with the open divertor. Radiation power in main region was also decreased in this case. These difference of radiation power from the main plasma between open and W-shaped divertor was not large enough to account for the apparent reduction of threshold power. In this study, the edge plasma parameters which relate to L-H transition were put emphasis on. It was found that edge ion temperature at which L-H transition occurred in the open divertor case was established by lower input power since edge density became smaller in the W-shaped divertor case under the condition of the same line-averaged density. After the modification of divertor geometry, ion edge collisionality just before L-H transition became lower and it was established with lower auxiliary power input. It was suggested that reduction of edge ion collisionality arose from neutral particles near the X-point by the analysis of poloidal profile of neutral density. Therefore, after the modification of divertor geometry, threshold power decreased though edge neutral density became larger.
IAEA 2001
