High-power and long-pulse injection with negative-ion-based neutral beam injectors in the Large Helical Device

TLDR

A negative‑ion‑based neutral beam injection system using large caesium‑seeded sources has operated reliably in the Large Helical Device since 1998, and its progress is reviewed from the perspective of negative ion source improvements. The study reviews the progress of negative‑NBI in the LHD, focusing on improvements to negative ion sources. Beam uniformity is improved by individually controlling local arc‑discharge with separate arc and filament power supplies, and the injection duration has been extended to 120 s, sustaining LHD plasma with low power from a single cooled‑grid ion source. Injection power reached 13.1 MW with three injectors, and a single injector achieved 5.7 MW at 184 keV—exceeding design limits—showing that negative‑ion‑based NBI performance is comparable to conventional positive‑ion NBI.

Abstract

A negative-ion-based neutral beam injection (NBI) system, in which large caesium-seeded negative-ion sources are utilized, has operated reliably in the Large Helical Device (LHD) since it was operational in 1998. The injection power of the hydrogen beam has been increased up to 13.1 MW with three injectors. In one injector with modified ion sources utilizing a multi-slotted grounded grid, the injection power reached 5.7 MW with an energy of 184 keV, which exceeds the designed value of 180 keV and 5 MW. Individual control of the local arc-discharge with independently divided arc and filament power supplies is effective in improving beam uniformity in a large negative ion source. The injection duration has been extended to 120 s, during which the LHD plasma is sustained by the NBI alone with a reduced power of 0.2–0.3 MW using one ion source with a cooled plasma grid. These results demonstrate that the injection performance of the negative-ion-based NBI is comparable to that of the conventional positive-ion-based NBI. The progress of the negative-NBI in the LHD is reviewed in detail from the point of view of the improvement of the negative ion sources.