Status of Negative-Ion-Based Neutral Beam Injectors in LHD

Recent progress in LHD N-NBI system is described. The system was designed to provide 15 MW of port-through beam power by three Beam Lines (BL-1 to BL-3) with 180 keV hydrogen for 10 sec. However, starting from 3.2 MW injection by two beam lines in 1998, their performance has not yet satisfied all the specifications at the same time. This is because several difficulties were found in large negative ion sources while the specifications were determined based on the R and D results of small ion sources. Hence, continuous efforts have been done to improve the performance of negative ion source during the machine maintenance period between the experimental campaigns of LHD. As a result, the injection power has been increased year by year, and in the latest LHD experimental campaign the total port-through power of 13.1 MW was achieved, where R and D on the ion source of BL-1 was most successful. Two improvements have been done on the ion source of BL-1. One is the optimization of magnetic cusp structure by modifying the shape of plasma source, which increased the negative ion current. The other is the adoption of multi-slot aperture for the grounded grid, which reduces the conditioning time of the accelerator dramatically. As a result, the specifications of beam energy and port-through power were satisfied in BL-1 at the same time. The remaining problem is a pulse length which is still limited due to high heat load on the grounded grid