A negative-ion-based neutral beam injection (NBI) system has been operated reliably in the Large Helical Device (LHD) since it was operational in 1998. The injection power achieved is 13.1 MW with three injectors. In one injector with modified ion sources with the multi-slotted grounded grid, the injection power reached 5.7 MW with an energy of 184 keV, both of which exceed the designed values of 180 keV-5 MW. The individual control of the arc-discharge with the divided arc and filament power supplies is effective to improve the beam uniformity. The injection duration is extended to 120 sec with a reduced power of 0.2-0.3 MW using one ion source with the cooled plasma grid. The performance of the negative-NBI in LHD is reviewed with regard to the progress of the negative ion sources.

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We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H(-)) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H(-) density is monitored with a cavity ring-down system to identify the electrons in the negative charges. The electron-saturation current decreases rapidly after starting to seed Cs, and ion-ion plasma is observed in the extraction region. The H(-) density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H(-) charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H(-) density decreasing at the beam extraction is compensated by the electrons diffusing from the driver region. In the plasmas with very low electron density, the pre-sheath of the extraction field penetrates deeply inside the plasmas. That is because the shielding length in those plasmas is longer than that in the usual electron-ion plasmas, and furthermore the electrons are suppressed to diffuse to the extraction region due to the strong magnetic field.

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