Description
The reconsideration of stellarator–heliotron programs is discussed in light of a review of key highlights from the LHD (Large Helical Device) and the plans for the post-LHD project. The LHD, based on the heliotron concept and employing superconducting coils, has been in operation since 1998 and will complete its mission in 2025. The LHD project has significantly advanced plasma science across a broad range of fields, both theoretically and through experimental achievements. The steady-state sustainment of 2-keV plasmas for 50 minutes and the achievement of beta values as high as 5% have demonstrated the feasibility of disruption-free, stable plasmas. A unique feature of the LHD is its ability to produce plasmas that are resistant and resilient to external disturbances, enabling novel experiments such as impurity injection and the application of magnetic perturbations. Advanced diagnostics with high spatiotemporal resolution have clarified the roles of electric fields and non-local effects in transport mechanisms. These physics lessons accumulated over 27 years will be transferred to the new machine, CHD-U (Compact Helical Device-U), a compact yet more flexible stellarator. The post-LHD project, including CHD-U, aims to explore micro-collective plasma phenomena, which are considered critical physics in the challenge toward a comprehensive understanding of toroidal plasmas. The knowledge and skills cultivated through the LHD project underpin this paradigm shift.