3-DIMENSIONAL ANALYSIS OF ION-BEAM DEFLECTION BY THE MAGNETIC-FIELD AT H- ION SOURCES FOR THE NEGATIVE-ION-BASED NEUTRAL BEAM INJECTION

In negative-ion-based neutral beam injection (NBI) systems for the lar ge helical device (LHD), beams must be transported over 13 m from the H- ion source to the injection port. In order to clarify beam deflecti on by the electron deflection magnets set in a beam extraction grid (E G) and to control beam transport direction, we analyzed beam trajector ies. The physics of the beam deflection was studied with theoretical c alculations and the deflection angle was estimated by 3D beam trajecto ry simulation. The evaluated deflection angle was 10 mrad in the oppos ite direction of the electron deflection when the maximum magnetic fie ld on the beam axis was 480 G and the beam energy was 83.2 keV. The el ectrostatic lens effect on the beam deflection at the EG exit was esti mated to be larger than the magnetic field effect. This deflection was reduced to 2 mrad by a 1.3 mm displacement of the grounded grid (CG) aperture, a result in agreement with experimental results of a 1/3-sca le model for the LHD ion source. The maximum GG aperture displacement of the LHD ion source was designed as 3.4 mm to reduce the deflection and to focus multibeamlets using the simulation. We have developed the ion source with this design. The targeted performance is a production of H- beams of 40 4 (10 mA/cm(2)), 180 keV. (C) 1998 American Institu te of Physics.