Goal and Achievements of Large Helical Device Project

TLDR

The Large Helical Device is a heliotron system using large superconducting magnets to study net‑current‑free plasmas, and its engineering and operational experience provide a technological foundation for fusion reactors. The experiment seeks to demonstrate high‑performance helical plasmas in a reactor‑relevant regime and to deepen scientific understanding toward a helical fusion reactor. Over 12 years, LHD has accumulated a physics database and operational experience, demonstrating stable steady‑state operation with 56,000 h of cryogenic uptime, a 2.96 T field, 23 MW neutral‑beam injection, 3 MW ion‑cyclotron resonance, 2.5 MW electron‑cyclotron resonance heating, achieving high beta 5.1 %, high density 1.2 × 10²¹ m⁻³, and 3200 s steady‑state at 490 kW.

Abstract

The Large Helical Device (LHD) is a heliotron-type device employing large-scale superconducting magnets to enable advanced studies of net-current-free plasmas. The major goal of the LHD experiment is to demonstrate the high performance of helical plasmas in a reactor-relevant plasma regime. Engineering achievements and operational experience greatly contribute to the technological basis for a fusion energy reactor. Thorough exploration for scientific and systematic understanding of the physics in the LHD is an important step to a helical fusion reactor. In the 12 years since the initial operation, the physics database as well as operational experience has been accumulated, and the advantages of stable and steady-state features have been demonstrated by the combination of advanced engineering and the intrinsic physical advantages of helical systems in the LHD. The cryogenic system has been operated for 56 000 h in total without any serious trouble and routinely provides a confining magnetic field up to 2.96 T in steady state. The heating capability to date is 23 MW of neutral beam injection, 3 MW of ion cyclotron resonance frequency, and 2.5 MW of electron cyclotron resonance heating. Highlighted physical achievements are high beta (5.1%), high density (1.2 × 1021 m−3), and steady-state operation (3200 s with 490 kW).