Origin of Non-Uniformity of the Source Plasmas in JT-60 Negative Ion Source

Abstract

In order to investigate the origin of the non-uniformity of the source plasmas in the JT-60 negative ion source, the distribution of H⁺ ions and H⁰ atoms, which were converted to H⁻ ions on the cesium covered surface, were measured by Langmuir probes and emission spectroscopy. Both of H⁺ ions and H⁰ atoms at the top and bottom of the ion source were about twice higher and lower than those in the center region, respectively. In particular, they were highly localized near the side wall of the top of the ion source. The analyses of the primary electron trajectories showed that this non-uniformity was due to the B x grad B drift of primary electrons emitted from filaments. These results suggest that the non-uniformity of the negative ions results in a locally poor beam optics and significant direct interception of the H⁻ ions on the acceleration grid. This is the reason why the negative ion beams are non-uniform in the JT-60 negative ion source.