Operation of the negative ion-based neutral beam injection system during Large Helical Device experimental campaigns

The Large Helical Device (LHD) is the only machine that relies upon negative ion-based neutral beam injection system (NBI) as the principal heating power source. Since the first injection in 1998, NBI has been used to heat over 3000 shots for 2 years. Performance has progressed step by step to reach an injection power of 4.5 MW, with beam energy up to 165 keV, and pulse lengths up to 80 s at 0.5 MW. System operating level has reached ∼30% of the design power and ∼65% of the H− design current. This tangential high-energy NBI system based on negative ions has proved to be as efficient a heating system in a helical device as in a tokamak. The ratio of injection neutral beam power to high voltage drain power (injection power efficiency) was typically about 0.3. This is a low efficiency compared to the design one. For long pulse injection beam blocking (such as observed on PLT) was encountered, but it did not limit the operation. In order to achieve the design output of 15 MW, a great R&D effort is underway on the negative ion test stand to improve the performance of 40 A 180 keV negative hydrogen ion source.