Challenges toward improvement of deuterium-injection power in the Large Helical Device negative-ion-based NBIs

Improvement of deuterium injection power in the negative-ion-based NBIs (n-NBIs) for the Large Helical Device (LHD) are reported. Co-extracted electron current at acceleration of deuterium negative ions (D- ions) limits the injection power. The electron current is reduced by decreasing the extraction gap, and the injected D- current evaluated from the injection power increased from 46 to 55 A. Greater electron reduction was achieved by installing a structure named an 'electron fence' (EF), with which D- beam power was successfully improved from 2.0 MW to 3.0 MW. The injection power in three configurations - without EF, with EF of 5 mm and 7 mm distance from the plasma grid (PG) surface - have been compared in both cases of hydrogen and deuterium operations, and it was found that the configuration with the EF of 5 mm distance was the best to satisfy the performance for both of hydrogen and deuterium injections. Although the co-extracted electron current is reduced in the negative ion sources applied for JT-60SA and ITER by utilizing the PG filter, it is possible to achieve more effective electron reduction by combining the PG filter and the EF.