Overview of the Large Helical Device project

TLDR

The Large Helical Device began plasma confinement experiments after eight years of construction, igniting its first plasma on 31 March 1998 with a 1.5 T magnetic field to gain experience with its superconducting coils. Two experimental campaigns were conducted in 1998, first using 84 GHz and 82.6 GHz electron cyclotron heating at 0.35 MW, followed by a second campaign employing 3 MW neutral beam injection. Magnetic flux mapping revealed nested magnetic surfaces, and the second campaign achieved electron densities up to 6 × 10^19 m⁻³, central temperatures of 1.4–1.5 keV, stored energies of 0.22 MJ, with energy‑confinement scaling matching the ISS95 law with slight improvement.

Abstract

The Large Helical Device (LHD) has successfully started running plasma confinement experiments after a long construction period of eight years. During the construction and machine commissioning phases, a variety of milestones were attained in fusion engineering which successfully led to the first operation, and the first plasma was ignited on 31 March 1998. Two experimental campaigns were carried out in 1998. In the first campaign, the magnetic flux mapping clearly demonstrated a nested structure of magnetic surfaces. The first plasma experiments were conducted with second harmonic 84 and 82.6 GHz ECH at a heating power input of 0.35 MW. The magnetic field was set at 1.5 T in these campaigns so as to accumulate operational experience with the superconducting coils. In the second campaign, auxiliary heating with NBI at 3 MW has been carried out. Averaged electron densities of up to 6 × 1019m-3, central temperatures ranging from 1.4 to 1.5 keV and stored energies of up to 0.22 MJ have been attained despite the fact that the impurity level has not yet been minimized. The obtained scaling of energy confinement time has been found to be consistent with the ISS95 scaling law with some enhancement.