Development of intense hydrogen-negative-ion source for neutral beam injectors at NIFS

Intense hydrogen-negative-ion source development, conducted at National Institute for Fusion Science (NIFS), is reviewed. Presently, the developed negative-ion sources are utilized in the negative-ion-based neutral beam injectors, which are installed to the Large Helical Device, the world’s largest superconducting fusion machine, and the total injection power has achieved 16MW with the energy of 180-190 keV using three injectors with six sources. In the developed negative-ion accelerator with multi-slotted grounded grid, the grid heat load is much reduced due to its high transparency, leading to a high-energy acceleration of a high-current negative ion beam. As a result, one ion source produces 190keV-37A of negative ions for 1.6sec at maximum, corresponding to 340A/m² of the current density. For further improvement of the negative ion source, plasma characteristics are investigated in the extraction region with a multi-diagnostics system. With the Cs seeding, the H⁻ density increases and the electron density decreases, and, finally, an ion-ion plasma which consists of almost positive and negative ions is observed. The measured negative ion density is not largely decreased toward the plasma grid surface, on which the negative ion is produced. Reduction of the negative ion density is observed by the negative ion extraction, and invasion of the electric field for the negative ion extraction is recognized. Understanding of the negative ion transport in the plasma and the mechanism of the negative ion extraction should contribute to improvement of the source performance.