Divertor plasma and neutral particles behavior under the local island divertor configuration in the Large Helical Device

In the Large Helical Device (LHD), the local island divertor (LID) experiment has been conducted to investigate how to improve the core confinement with edge plasma control. An advantage of the LID configuration over the intrinsic helical divertor is the high pumping efficiency with the closed divertor module (neutralizer and pumping duct) and high speed pump-system. The particle evacuation efficiency, evacuated particle number/fueled particle number, has been estimated in experiment. This efficiency depends on the relative position of the closed divertor module (divertor head) to m/n = 1/1 magnetic island’s outer separatrix corresponding to the divertor leg, and the efficiency is revealed to be 0.6–1. A new operational regime with high core density (>4 × 1020 m−3) and a strongly peaked density profile has been discovered in the LID discharges. It is considered that a large effective pumping efficiency is necessary to enter this operational regime.