Multibeamlet focusing of intense negative ion beams by an aperture displacement technique

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Abstract
Multibeamlet focusing of an intense negative‐ion beam has been performed using beamlet steering by aperture displacement. The apertures of the grounded grid were displaced as all 270 beamlets (18×15) in an area of 25 cm×26 cm are steered to a common point (a focal point) in both the two‐stage and the single‐stage accelerators. The multibeamlets were successfully focused and the e‐folding half width of 10 cm was achieved 11.2 m downstream from the ion source in both accelerators. The corresponding gross divergence angle is 9 mrad. The negative‐ion beamlets are deflected by the electron deflection magnetic field at the extraction grid and the deflection direction reverses line by line, resulting in the beam splitting in the deflection direction. This beamlet deflection was well compensated also using beamlet steering by the aperture displacement of the grounded grid. The beam acceleration properties related to the beam divergence and the H− ion current were nearly the same for both the two‐stage and the single‐stage accelerators, and were dependent on the ratio of the extraction to the acceleration electric fields.

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