Progress of neutral beam injection system on JT-60U for long pulse operation

Recent progresses of NBI system on JT-60U are presented with emphasis on extension of pulse duration from 10 sec to 30 sec. The positive-ion-based NBI (P-NBI) unit has extended its duration up to 30 sec with 2 MW at 85 keV by modifying the electric power supplies, the beam limiters, and the control system. The negative-ion-based NBI (N-NBI) unit, whose target beam energy is 500 keV for 10 see, has demonstrate its capability up to 25 sec with similar to 1 MW at 350 keV. The progress of the pulse duration has contributed to study a quasi-steady-state plasma, where the NBI pulse duration is significantly longer than the plasma current diffusion time. Moreover, it has been found that the thermal time constants of the main key components with active cooling, such as the ion source of the present NBI system, are less than similar to 15 sec. These operational results have beneficial effects on the design of the upgraded NBI system for the modified JT-60U with superconducting coils (so called NCT), where the NBI pulse duration of 100 sec is required.