Speaker
Description
Deuterium beam operation on neutral beam injectors (NBI) have been carried out successfully in the Large Helical Device (LHD) in 2017. We had operated three hydrogen negative-ion and two positive-ion based NBI’s since 1998 and 2005, respectively. In 2017, the injection beam power has reached 31 MW used five beam lines, which gave us a new operation region for high ion temperature plasma. Upgrading the beam energy from 40 keV to 60 keV and from 40 keV to 80 keV for deuterium operation in the positive ion based NBI of BL4 and BL5, respectively, we have achieved over 9 MW beam injection which is exceeded designed values. We have shown high-arc efficiency (extracted beam current/arc power) with 83% high deuteron ratio in the positive source measured by Doppler spectroscopy. We also extracted deuterium negative ion beam from three tangential NBI’s (BL1, BL2, and BL3) without increase beam energy, although the injection power decreased by half. There was almost no contamination of hydrogen due to the switching to the deuterium beam. The arc efficiency of the deuterium negative ion is 60 - 70 % of the hydrogen case, and the current density of the accelerated current reached 190 A/m$^2$, which is comparable with the designed value of deuterium beam current for ITER-NBI. Since the deuterium beam width observed on the calorimeter is widened by 1.2 times, it might be considered that the geometric focusing condition of the beam is not optimum for deuterium. The beam injection efficiency decreased by 15 % caused by several factors. Certainly, the extracted electron current increased in deuterium operation, however, we have any trouble on the beam line components.
