First results of deuterium beam operation on neutral beam injectors in the large helical device

Deuterium beam operations on neutral beam injectors (NBI) have been carried out successfully in the Large Helical Device (LHD) in 2017. We injected a total of 31 MW neutral beam power, which gave us a new operation region for high ion temperature plasma. Upgrading the beam energy from 40 keV to 60 keV and then to 80 keV for deuterium operation in a positive ion based NBI (P-NBI) of BL4 and BL5, respectively, we have achieved 10.6 MW (75 keV) beam injection in the P-NBI. We confirmed a good arc efficiency (extracted beam current/arc power) with 83% high deuteron ratio which is the same as the hydrogen operation in the positive source. We also extracted deuterium ion beam from three negative-ion based NBIs (N-NBI) without increase beam energy, although the injection power decreased by less than half. The averaged arc efficiency of the deuterium negative ion is approximately 66% of the hydrogen case, and the current density of the accelerated beam current reached 190 A/m². We have clearly found the lower limit of electron current ratio depending on the arc discharge power. Certainly, the extracted electron current ratio increased in deuterium operation. However, we have had no trouble on the beam line components.