Stability of High Power Beam Injection in Negative-Ion-Based LHD-NBI

We describe the characteristic of stable beam injection in a neutral beam injector (NBI) for Large Helical Device (LHD) in high injection power of more than 6 MW. In the NBI, it takes a week after starting Cs seeding to finish the pre-injection conditioning. The injection starts with the beam power similar to 6.2 MW, and the maximum power reaches similar to 7 MW. The Cs-seeding rate affects the beam stability in such high power injections. By optimizing the rate to 0.65 mg / shot, the success ratio, which is defined as a ratio of actual pulse duration to setting one, increases to 85 - 90 % in the power and energy range of more than 6.2 MW and 185 keV, respectively. The weights of Cs adsorbed on several surfaces in the ion sources of the NBI are measured by means of Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), and averaged surface densities are calculated by dividing with the several surface areas. The seeded Cs of 99.5 % is condensed in the plasma generator, and very tiny amount of Cs reaches the surfaces of the accelerator grids. This very low amount of Cs on the grids is interpreted that most of the Cs atom evaporated from the inner walls is ionized during the arc discharges, and repelled to the source plasmas by the electrostatic field for H-extraction.