Recipe of helium neutral beam injection for studying helium transport in fusion plasma

In order to accelerate the understanding of helium transport physics, the helium neutral beam experiment started in Large Helical Device (LHD) from the FY2021 experiment. We have modified a pumping system and Neutral Beam (NB) injection system to study helium transport in a typical fusion experimental device. A positive-ion-source-based NB for hydrogen and deuterium beam has been modified to inject helium NB into a plasma. Also, a pumping system was enhanced to increase the helium pumping rate for the vacuum vessel. In the NB, the gas supply system was modified to puff helium and argon gas, enabling a highly focused 78 keV helium-NB to inject into the plasma. In the pumping system, the turbo molecular pumps were added to increase the helium pumping speed, and a factor of approximately 1.5 successfully increased. It was observed that the arc current of the NB ion source decreased during the helium-NB operation. The cause was discovered to be the production of helium bubbles on the surface of tungsten filaments, and the recovery way using argon discharge was demonstrated. In the helium transport experiment using the helium-NB at LHD, measurements of energetic-helium-ion and bulk-helium-ion were successfully performed, and their distributions were observed.