100 seconds and megawatt negative ion beam accelerations at the CRAFT NNBI test facility

Abstract A promising negative-ion-based neutral beam injection (NNBI) system should achieve high energy, high power and long pulse simultaneously for the plasma heating and current drive in the large-scale fusion devices. A NNBI test facility has been constructed in the Comprehensive Research Facility for Fusion Technology (CRAFT) in China. The installed capability of the whole CRAFT NNBI test facility is the beam energy of 400 keV, the ion beam current of 28 A, and the continuous beam pulse of 1 h. A dual-driver RF negative ion source (beam size: 0.75 × 0.3 m 2 , design acceleration voltage: 200 kV) has been developed and tested for the first operation of the test facility. Several significant improvements were applied to the test facility or the negative ion source during the system maintenance. In the second experimental campaign of CRAFT NNBI test facility, beam extraction and acceleration have reached 100 s duration and megawatt power levels. The typical values were 135 keV, 10.6 A (≈180 A m −2 ) and 110 s at the filling pressure of 0.4 Pa. For ten second beam pulses higher energy and current levels were established (173 keV and 12 A). The results demonstrated the negative ion source can reliably operate for long pulse. Some problems about the high-voltage holding, particle and heat flux were revealed in the high-power and long-pulse beam acceleration.