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

Origin of the longitudinal beam non-uniformity, that is one of the key issues in large Cs-seeded negative ion sources for fusion application, was experimentally investigated in the JAERI 10 A negative ion source. After a sufficient caesium of ∼0.3 g was seeded in the negative ion source to enhance the negative ion production, the longitudinal distribution of the beam intensity was measured. The distribution of the beam intensity was non-uniform, and was absolutely 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 caused by B x ∇B drift of the fast electron from filaments. Here, B is the magnetic field near the filaments whose strength is the sum of the magnetic filter field and the confinement magnetic filed around the negative ion source. To improve the beam non-uniformity, the B x ∇B drift of the fast electrons from filaments was suppressed by modifying the filament configuration. The beam non-uniformity was dramatically improved. The root-mean-square deviation of the beam intensity from the averaged value deceased to a half of that before the modification while the beam intensity integrated along the longitudinal direction was kept to be constant. Thus it was experimentally confirmed that the beam non-uniformity was improved by suppressing the B x ∇B drift of the fast electrons from the filaments. (author)