Improving the performance of a negative-ion based neutral beam injector for JT-60U

Yamamoto, Takumi; Umeda, Naotaka; Kuriyama, Masaaki; Lei, G.*; Grisham, L. R.*; Kawai, Mikito; Oga, Tokumichi; JT-60 NBI-Team

The performance of a 500keV negative-ion based neutral beam injection system for the JT-60U has been enhanced significantly. By improving the beam divergence, 10-second operation of beam injection has been attained. Small peaks were observed in the beam profile measured at the near field. These peaks are clearly resulted from the deflection of the beamlets due to an unwanted electric field. Correction of the deflection reduced the heat load on the limiter at the injection port by more than 50%. The Doppler shifted spectrum from the negative ion beam was measured, and then indicated that electron stripping of the negative ion beam occurs primarily inside the extraction grid, as expected from theoretical consideration. The electron stripping contributed to half of the heat load on the grounded grid.