Recycling control by divertor pumping in the closed helical divertor of LHD

Controlling hydrogen fuel particles is critical for achieving a steady-state operation in magnetic confinement fusion devices. In the Large Helical Device, efforts to control wall recycling have led to the development of a closed helical divertor (CHD) with integrated in-vessel pumping systems. This paper review presents the conceptual evolution, engineering implementation, and experimental validation of the CHD, highlighting its role in enhancing neutral particle compression and recycling control. By applying both cryo-sorption and non-evaporable getter pumps within the CHD, significant improvements in neutral pressure compression, density control, and impurity exhaust were demonstrated. These developments enable access to control the recycling, improve energy confinement, and operational flexibility during long-pulse discharges. The results provide a solid experimental foundation for integrating advanced divertor pumping strategies into future steady-state fusion reactors.