Development of a focal guidance system for microbeam formation

Abstract

We developed a focal guidance system based on a thin lens approximation to support beam-focusing during microbeam experiments in which the only input parameters are the geometry, lens layout, and lens parameters of doublet-to-quadruplet systems in various couplings. The software separately calculates the X-and Y-axis magnetic fields at convergence, beam traces of any magnetic fields, and matrix elements. The operator does not need to have expertise in beam transport. The software reveals how a beam is focused, and the beam behavior as the magnetic field changes. The parasitic aberration coefficient, which is related to the excitation error, was calculated for an automatic focusing system developed at Tohoku University. Calculations were performed for typical doublet-to-quadruplet lens systems; the magnetic fields at convergence were within 1% of the values calculated by the transport code. In real-world experiments, it is relatively easy to focus the beam on only one axis. By normalizing the X and Y focal lines to the excitation currents in the X focus and Y focus, respectively, the currents for both axes focus are obtained and the user can thus understand how to change the excitation current. This is necessary for beginners who require guidance on the direction in which the excitation current should be changed. Our software will be available from http://web.tohoku.ac.jp/fnl/.