The influence of accelerator parameters on the performance of submicron quadrupole probe-forming systems
This paper examines the role of the accelerator as a component in a MeV ion microprobe system. The aberrations in the probe-forming lens are first discussed, so that the critical beam properties can be specified. It is found that, at least in the triplet configuration, aberration due to sextupole contamination of the quadrupole field is the dominant effect limiting the acceptance of the final lens and, unless this is eliminated, chromatic effects due to momentum spread in the beam are insignificant. A novel quadrupole lens has been developed which has no measurable sextupole field component, and using these lenses the acceptance is limited by 5th- and higher-order effects, although in submicron operation these are small compared with external influences such as mechanical vibration and scattering at slit edges. The current in the final spot is determined by the brightness of the beam. It is predicted and confirmed by measurement that the stripper in a tandem degrades the beam brightness. It is concluded that single-ended machines should be better for microbeam operation, but that for resolutions down to 1 μm either type is suitable, and the choice is influenced by other considerations.