Oscillation of negative ion beamlet and effect on beam focusing for neutral beam injection

Negative-ion-based neutral beam injection (negative NBI) is a power tool for heating magnetically confined plasmas and current drives. An RF negative NBI is being developed for ITER. Improving beam focusing is an urgent issue in the ITER-NBI development. We investigated the RF effect on beam focusing using the NIFS-NBTS facilities and found that the beamlet width oscillates with the RF frequency applied near the beam extraction region. The oscillation of the beamlet axis position was also observed, which degraded the beam focusing. The perveance matching condition and lower RF frequency can suppress these beamlet oscillations. This presentation will report the detailed setup of the experiment and the experimental results, then emphasize the contribution to the ITER-NBI development.