Compensation of beam deflection due to the magnetic field using beam steering by aperture displacement technique in the multibeamlet negative ion source

A beam steering technique using aperture displacement was examined to corre ct the negative ion beam deflection due to the magnetic field for electron suppression in a large-area multibeamlet H- source. The total deflection an gle was estimated, including the effect of the deflection by the electron s uppression magnetic field and the beam steering by the aperture displacemen t, both by linear optics theory and by three-dimensional beam trajectory si mulation. Two methods were compared; one used the displacement both of the grounded grid (GG) apertures and of the exit part of the extraction grid (E G exit) apertures, and the other used the displacement only of the EG exit. The beam steering experiments were performed using a large-area multibeaml et H- source with both displacement methods, and the results were compared with the theoretical estimations. As a result, both methods were effective to correct the beam deflection. In particular, the displacement of only the EG exit with a simplified displacement structure achieved a large steering angle by a small displacement. The steering angle in the experiment was a few mrad smaller than the estimations. Based on these results, the aperture displacement of the EG exit was applied to the 1/5 segment of a H- source in the Large-Helical-Device neutral beam injector, where the GG apertures a re displaced only focusing of the large-area multibeamlet. In this case, 1. 0 mm of the displacement is concluded to be proper to sufficiently compensa te the beam deflection at 180 keV, from extrapolation of the beam energy ch aracteristics at 100-140 kev. (C) 2001 American Institute of Physics.