High Power Neutral Beam Injection in LHD

The results on high power injection with the neutral beam injection (NBI) system for the Large Helical Device (LHD) are reported. The system consists of three beam-lines with two hydrogen negative ion (H− ion) sources installed in each beam-line. In order to improve the injection power, a new beam accelerator with a multi-slot grounded grid (MSGG) has been developed and applied to one beam-line. Using the accelerator, a maximum power of 5.7 MW was achieved in 2003 and 2004, and the maximum energy of 189 keV was reached. The power and energy exceeded the design values of the individual beam-line for LHD. The other beam-lines also increased their injection power up to about 4 MW, and the total injection power of 13.1 MW was achieved with three beam-lines in 2003. Although the accelerator had an advantage in high power beam injection, extracted beams expand in the short side direction of the ground-grid slot. The disadvantage has been resolved by modifying the aperture shapes of the steering grid.