The (LHD) was a fusion research device located in Toki, Gifu, Japan. It was operated by the National Institute for Fusion Science (NIFS) until 2025. The LHD employed the heliotron magnetic field configuration originally developed in Japan. It was the world's largest and most powerful superconducting stellarator. In 2015, Wendelstein 7-X began operation with equal plasma volume but a weaker magnetic field. Until that time, LHD was also the largest superconducting magnetic confinement fusion device of any kind, including tokamaks.
The objective of the project was to conduct fusion plasma confinement research in a steady state in order to elucidate possible solutions to physics and engineering problems in helical plasma reactors.
The LHD used neutral beam injection, ion cyclotron radio frequency (ICRF), and electron cyclotron resonance heating (ECRH) to heat the plasma, much like tokamaks. The helical divertor heat and particle exhaust system used the large helical coils to produce a diverting field. This configuration allows for the modification of the stochastic layer size, which is positioned between the confined plasma volume and the field lines that terminate on the divertor plate. Boundary plasma research at LHD focused on the capability of the helical divertor as an exhaust system for heliotrons and stellarators.
LHD achieved the longest sustained runs of any magnetic fusion experiment, stellarator or tokamak, at 48 minutes with 1.2x1019m-3 plasma density and 2 keV plasma temperature, and 54 minutes with 0.4x1019m-3 plasma density and 1 keV plasma temperature.
Research into the heliotron stellarator type with the aim of generating energy from fusion continues in Japan with the company Helical Fusion, founded by NIFS scientists. The company was conditionally selected for support from the Japanese government for its fusion projects in 2026.
History Design finalized 1987 Start of construction 1990 Plasma operations from 1998 Neutral beam injection of 3 MW was used in 1999. In 2005 it maintained a plasma for 3,900 seconds.Achievement of One Hour Discharge with ECH on LHD 2005 In 2006 a new helium cooler was added. Using the new cooler, by 2018 a total of 10 long term operations have been achieved, reaching a maximum power level of 11.833 kA. To aid public acceptance, an exhaust system was designed to catch and filter the radioactive tritium the fusion process produces. The Large Helical Device experiment was concluded on Christmas Day of 2025 after more than 27 years of operation.
See also Fusion reactor National Institutes of Natural Sciences, Japan
References
External links Large Helical Device Website Super Dense Core plasmas in LHD. Harris. 2008 16 slides. advanced - inc ballooning mode and future development options