Substantial reduction of H-mode threshold power with W-shaped divertor
The H-mode is generally considered an improved confinement mode used for the standard operation scenario of the reduced cost ITER. In order to realize the burning plasmas, sufficient heating power into the plasma should be applied externally to produce the H-mode transition.
In JT-60U, the divertor geometry was modified from the open divertor to a new W-shape pumped divertor in June 1997 to improve the controllability of heat and particles. Recently, a systematic survey of the power necessary for H-mode transition has been carried out in JT-60U. The results show that the threshold power for H-mode transition is reduced by more than 30%, in comparison with the results from open divertor experiments (see figure below). The reduction of the threshold power could be resulted from the suppression of back flow of neutral particles and impurities from the divertor region to the core plasma.
This new result obtained under the W-shaped divertor configuration relevant to ITER is expected to contribute to ITER physics R&D.
Fig. Density dependence of the H-mode transition threshold power, normalized by the magnetic field strength