Initial physics achievements of large helical device experiments

TLDR

The study aims to investigate currentless, disruption‑free steady plasmas in a large‑scale heliotron configuration to advance toroidal plasma physics. The fully superconducting Large Helical Device, with a 3.9 m major radius and 0.6 m minor radius, began operations after eight years of construction and now conducts experiments on large‑scale heliotron plasmas. Magnetic mapping confirmed nested, healthy surfaces, and 3 MW neutral‑beam heating produced a fusion triple product of 8 × 10¹⁸ keV m⁻³ s, electron and ion temperatures of 1.5 ke.

Abstract

The Large Helical Device (LHD) experiments [O. Motojima, et al., Proceedings, 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437] have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas (R=3.9 m, a=0.6 m), which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection (NBI) has produced plasmas with a fusion triple product of 8×1018 keV m−3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to 〈β〉=0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics (MHD) instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.