Behavior of multiple Gaussian beam components in a single negative ion beam aimed at fusion plasma heating*

  Negative ion beams are utilized for advanced applications such as accelerators of particle physics and medical fields, and plasma experiments in nuclear fusion. It is because their neutralization efficiency is higher than that of positive ion beams. Well focused negative hydrogen and/or deuterium ion beams are required for neutral beam injection (NBI) systems for heating magnetically confined fusion plasmas and current drive. The study of a single negative ion beam clarified the detailed structure and divergence of the beam. Our research group with the National Institute for Fusion Science found that the single negative ion beam consists of multiple Gaussian beams, which have been measured with a pepper-pot type emittance meter. The peak positions of the components shift with the change in the acceleration voltage while keeping the extraction voltage constant. It implies that the shift of multiple Gaussian beam components can affect the resultant superimposed single negative ion beam's divergence. This presentation reports the spatial distributions of each component in a plane perpendicular to the beam direction, and their abundance ratio while changing the arc power. The abundance ratio could be used for the investigation of relative contributions of negative ions caused by the surface production at a plasma grid region.