Optimization of the radial magnetic field of an 18 GHz electron cyclotron resonance ion source at the Heavy Ion Medical Accelerator in Chiba

Since the narrowly concentrated ion-density distribution enhances the space charge effects, the extracted beam intensity is lower than case with a uniform distribution. In order to realize the uniform ion-density distribution at the extraction aperture for the higher beam intensity, the radial magnetic field given by the permanent sextupole magnet has been optimized. The ion-density distribution estimated by the TRAPCAD code depends on the magnetic trap configuration. The diameter of the electron cyclotron resonance (ECR) zone determined by the inner diameter of the sextupole magnet was enlarged, and consequently the density of the flux lines running through the ECR zone decreased at the extraction aperture. The ion-density distribution could become more uniform. The inner diameter of the sextupole magnet of an 18 GHz ECR ion source for the Heavy Ion Medical Accelerator in Chiba at the National Institute of Radiological Sciences was increased from 46 to 66 mm, so that the extracted intensities of Ar8+ and Ar9+ ions increased from 250 and 150 e mu A to 1100 and 650 e mu A, respectively. (C) 2000 American Institute of Physics. [S0034-6748(00)64202-8].