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,

The H− ion production and transport processes are numerically simulated to clarify the origin of H− beam nonuniformity. The three-dimensional transport code using the Monte Carlo method has been applied to H0 atoms and H− ions in the large “JAERI 10A negative ion source” under the Cs-seeded condition, in which negative ions are dominantly produced by the surface production process. The results show that a large fraction of hydrogen atoms is produced in the region with high electron temperature. This leads to a spatial nonuniformity of H0 atom flux to the plasma grid and the resultant H− ion surface production. In addition, most surface-produced H− ions are extracted even through the high Te region without destruction. These results indicate a correlation between the production process of the H− ion and the spatial nonuniformity of the H− ion beam.

The influence of magnetic filter configuration on the beam uniformity was examined to improve beam uniformity in a large Cs-seeded negative-ion source. By reducing the filter strength of the transverse magnetic field used in a typical negative-ion source, the beam uniformity was largely improved with the improvement of the plasma uniformity while the beam intensity was kept to be nearly constant. However, the coextracted electron current greatly increased. To suppress the coextracted electron current, a tent-shaped magnetic filter was applied together with modifications in the cusp magnets to form a typical multicusp positive-ion source arrangement. The uniformity in longitudinal beam profile was improved with the deviation of local beam intensity within 16% that was nearly equal to the deviation obtained at 50Gcm of the transverse filter strength. In the meantime, the coextracted electron current was kept to be the same as the H− ion current. The present result suggests that the uniformity of H− ion-beam profile is affected by the uniformity of atoms or protons in the source plasma, which are the primary species of negative ions under Cs-seeded surface production.

R&D pertaining to a 1 MeV accelerator and a large negative ion source has been carried out at the Japan Atomic Energy Research Institute for the ITER neutral beam system. The R&D is, at present, progressing towards: (1) 1 MeV acceleration of H− ion beams at the ITER relevant current density of 200 A m−2 and (2) improvement of uniform negative ion production over wide extraction area in large negative ion sources. Recently, H− ion beams of 1 MeV and 140 mA level have been generated with a substantial beam current density (100 A m−2). In the uniformity study, it was clarified that the electron temperature in the ion extraction region was locally high (>1 eV), which resulted in the destruction of negative ions at a high reaction rate. Interception of fast electrons leaking through a transverse magnetic field called ‘magnetic filter’ has been found to be effective in lowering the local electron temperature, followed by an improvement of negative ion beam profile.

No abstract