Characterisation of lens aberrations and parasitic fields using beam rocking

Abstract

Beam rocking has been used on both the Oxford high-excitation triplet and the Melbourne Russian quadruplet to study lens aberrations such as multipole fields. A focused MeV proton beam is rocked over a fine mesh grid and the resulting beam displacement is mapped. Distorted patterns are produced in the presence of multipole fields originating either in the lenses or elsewhere due to stray fields. The minimisation of parasitic multipole fields is not only required in order to focus ion microprobes to small spot sizes, but is also important for the production of proper angular channelling scans using beam rocking since such fields distort an otherwise linear scan. In the absence of sextupole and higher order multipole fields, symmetric patterns are produced, exhibiting only spherical aberration. The patterns obtained experimentally are interpreted with the aid of simulations produced using an ion optics ray-tracing program.