Study of the extracted beam and the radial magnetic field of electron cyclotron resonance ion source at HIMAC

Experimental results on an 18 GHz electron cyclotron resonance (ECR) ion source (NIRS-HEC) and a 10 GHz ECR ion source (NIRS-ECR) at National Institute of Radiological Sciences (NIRS) show that an extracted beam intensity strongly depends on the radial magnetic field distribution generated by a permanent sextupole magnet. In order to understand these results, we simulated beam extraction under strong influences of space charge. In the simulation, a current intensity at a different position of an extraction slit is assumed to be roughly proportional to a corresponding transverse area of the ECR zone under the assumption that ions are extracted along a longitudinal magnetic flux line. The calculations show that an optimum value of the sextupole magnetic field strength may exist for a given extraction configuration and beam intensity. Based on the simulation, beam intensity measurements have been performed with six sextupole magnets having different magnetic field strengths for two ion sources, and the experimental results are consistent with the calculations.