Speaker
Description
The Large Helical Device (LHD) experiments will conclude in December 2025, marking nearly three decades of divertor research. This milestone provides a unique opportunity to summarize the achievements and lessons learned from the development of the helical divertor concept and its role in steady-state stellarator operation.$\newline$
Early LHD experiments demonstrated that the open helical divertor produced neutral pressures below 0.1 Pa, leading to confinement degradation and limitations in long-pulse operation. To overcome these issues, the Closed Helical Divertor (CHD) was conceptually designed in the late 2000s using magnetic field-line tracing and three-dimensional neutral transport simulations with the EIRENE code. The CHD, fully implemented in the 2010s, introduced a geometrically closed divertor structure with integrated in-vessel pumping systems located in the inner toroidal sections. A combination of cryo-sorption and non-evaporable getter pumps provided a total effective pumping speed exceeding 80 m$^3$/s, enabling direct exhaust of more than 50% of the fueling particles [1]. This significantly reduced wall recycling, improved density controllability, and enabled long-pulse plasma operation.$\newline$
Systematic investigations of particle control during long-pulse discharges revealed the critical role of divertor pumping. In inward-shifted magnetic configurations ($R_{\mathrm{ax}}$ = 3.60 m), strong localization of particle flux was observed on the inboard side where the CHD modules were installed. Forty-second ECH-heated discharges demonstrated that divertor pumping suppressed wall saturation, maintained stable temperature profiles, and enabled superior density control. In contrast, discharges without pumping suffered from density rise and confinement degradation due to enhanced wall recycling [2]. Profile analyses showed higher core electron temperatures with pumping, exhibiting electron internal transport barrier (e-ITB)-like characteristics. Heat-conductivity evaluations confirmed that the confinement improvement was associated with reduced wall fueling and sustained low core thermal diffusivity.$\newline$
More recently, ultra-high neutral pressures up to 2.4 Pa—comparable to those in tokamaks—have been observed in $R_{\mathrm{ax}}$ = 3.55 m configurations. These regimes are associated with enhanced neutral compression, near-wall condensation, and radiation-driven detachment, achieved without significant degradation of core confinement [3].$\newline$
Overall, the LHD divertor program has established effective strategies for particle exhaust and recycling control in stellarators, providing a robust experimental foundation for steady-state reactor concepts. As LHD completes its final experimental campaign, its divertor studies leave a lasting legacy for next-generation devices such as W7-X and for the design of DEMO-class reactors.
[1] G. Motojima+, Nuclear Fusion, 59, (2019), pp. 086022.
[2] G. Motojima+, Phys. Scr. 97 (2022), pp.035601.
[3] U. Wenzel+, Nuclear Fusion 64 (2024), pp. 034002.