Recipe of Helium Neutral Beam Injection for Studying Helium Transport in LHD

To advance the understanding of helium transport physics, helium neutral beam (He-NB) experiments were initiated in the Large Helical Device (LHD) during the FY2021 experimental campaign. To facilitate these investigations, modifications were implemented in both the impurity exhaust system and the neutral beam (NB) injection system, enabling the use of the helium beam in a typical fusion experimental environment. In the positive-ion-source-based NB system—originally designed for hydrogen and deuterium beams—the gas supply lines were adapted to introduce helium and argon into the ion sources, while argon was injected into the neutralizer cells. These modifications allowed for the injection of a high current and high focused 78 keV He-NB into the plasma. To improve helium exhaust efficiency at main vessel, turbo-molecular pumps were installed, increasing the effective pumping speed by approximately 1.4 times. During He-NB operation, a gradual decrease in the arc current of the NB ion source was observed with each discharge. This effect was attributed to the formation of helium bubbles on the tungsten filament surface, and a recovery method using argon gas discharges was successfully demonstrated. Plasma heating by He-NB injection was confirmed, and both energetic helium ions and bulk helium ions were successfully measured. Their spatial distributions were also observed. These results contribute to a deeper understanding of helium behavior in fusion plasma and offer valuable insights for future helium transport studies.