The origin of beam non-uniformity in a large Cs-seeded negative ion source

The origin of longitudinal beam non-uniformity, which is one of the key issues in large caesium (Cs)-seeded negative ion sources for fusion application, was experimentally investigated in the JAERI 10 A negative ion source. A sufficient caesium was seeded in the negative ion source to enhance the negative ion production. The beam intensity with Cs was typically four times higher than that without Cs, and was relatively high in the upper region where the electron temperature and plasma density were relatively high. This distribution was significantly different from that without Cs. From the correlation between the beam intensity and the plasma parameters, it was found that the beam non-uniformity was due to the localization of the plasma. The front edge of the filaments was bent towards the plasma grid to suppress the plasma localization. This resulted in the significant improvement of the beam uniformity. The root-mean-square deviation of the beam intensity from the averaged value decreased to a half of that before the modification while the beam intensity integrated along the longitudinal direction was kept constant. This result indicates that the suppression of the plasma localization by the filament modification is effective for improving the beam uniformity in the Cs-seeded negative ion source.