Development of Automatic Beam Focusing System

Abstract

  We have developed an automatic beam-focusing system to reduce the experimental set-up time for IBA experiment using a submicron beam. Parasitic aberration due to misalignment and astigmatism must be eliminated or minimized to obtain an ideal beam spot size in these experiments, because astigmatism from excitation error and axial misalignment broaden the beam. The system involves a two-stage process of contrast and phase-difference detection that includes an automated minimization algorithm for astigmatism. The process can identify misalignments of axial rotation by measuring the line profiles of a fine mesh grid. The contrast method focused a proton microprobe to approximately 1 μm² within 30 minutes, without the need for manual control when rotational misalignment was not present, while the phase-difference method successfully reduced the rotational error of lenses. The system is now in operation at Tohoku University and a private company.