Reduction of L-H transition threshold power under the W-shaped pumped divertor geometry in JT-60U

A remarkable reduction of the L-H threshold power was observed in JT-60U under the W-shaped pumped divertor geometry, in comparison with the results for the previous open divertor. The density range was extended to 0.6nGWL ( = Ip/πa2), where a stronger than linear density dependence was found. A reduction of the radiation power and edge temperature as well as a nonlinear increase in edge density were also observed. The poloidal distribution of the neutral particle density was first examined in this work, and it was found that compression of neutral particles near the X-point or loss of homogeneity on a flux surface produced by the modification of the divertor geometry may be an influential determinant of the L-H threshold power.