A high-power negative-ion-based NBI system is now under construction for Large Helical Device (LHD), which is the world-largest superconducting helical device. The neutral beam injection power and energy are designed to be 15 MW and 180 keV, respectively, for hydrogen, and negative ions are used for the beam source because of their high neutralization efficiency in the designed energy range. Two beamlines with an injection power of 7.5 MW are installed to LHD, the injection direction of which is tangential and balanced with each other to cancel the beam-driven current. The beamline consists of two vacuum vessels, the ion-source vacuum vessel and the beam-dump vacuum vessel, connected with a 3-m long neutralizer. Two large negative-ion sources are attached side-by-side to the ion-source vacuum vessel, which is an expansion chamber for the gas pumping with a cryo-sorption pump of 360 m³/s. The beam-dump vacuum vessel contains a bending magnet, residual positive and negative beam dumps made of swirl tube and a calorimeter, and is evacuated with a cryo-sorption pump of 1000 m³/s to keep the gas pressure below 4 x 10^-3 Pa. The effective neutralization length is about 5 m and the neutralization gas is supplied additionally into the ion-source vacuum vessel to control the gas thickness for the neutralization.

The ion source, which is a Cs-seeded volume production type one, is designed to produce 180 keV-40 A negative ion beam with a current density of 40 mA/cm² for 10 sec. The arc chamber is 35 cmW x 145 cmL x 21 cmD, and is equipped with an external magnetic filter for the negative ion production. The beam area is 25 cm x 125 cm and divided into five sections. A three-grid single-stage accelerator is used and every beamlet is focused 13 m downstream by the aperture displacement technique. The prototype ion source has already started operation at the negative NBI teststand, and produced high-power negative ion beams.

The acceleration power supply provides 170 kV-90 A to two ion sources simultaneously, and the extraction power supply provides 15 kV-75 A to one ion source individually. The high-current arc power supply of 5 kA is used for the plasma production.

The fabrication of the beamline components and the power supplies have been almost finished. The installation to LHD will start in October and be complete in March, 1998, followed by the start of injection to LHD.