Ion-extraction performance of the circular magnetic-multipole bucket ion source applied for the neutral beam injection system of the Heliotron E machine

The ion-beam extraction performance of the 30-kV–35-A circular magnetic-multipole bucket ion source applied for the neutral beam injection system of the Heliotron E fusion machine has been investigated with special reference to ion-beam optics. By using the numerical simulation code for the cylindrical ion-extraction system, the computational beam-divergence angles have been compared with the experimental data. The experiments show that the applied three-electrode beam optics are satisfactorily stable against source-plasma variation and that the minimum beam-divergence angle is less than 1.15 deg. The obtained optimal perveance of the ion source is nearly 6.3×10−6 (A/V3/2), and the experimental optics behavior is in reasonable agreement with the numerical prediction. On the basis of the experiments, the effect of the edge angle of the first beam-forming, high-potential electrode is also discussed. The experiment suggests that the obtuse angle (∼73°) for that edge is preferable to the acute angle (∼30°).