Characterisation of lens aberrations and parasitic fields using beam rocking

Beam rocking has been used on both the Oxford high-excitation triplet and t he 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 t he presence of multipole fields originating either in the lenses or elsewhe re due to stray fields. The minimisation of parasitic multipole fields is n ot only required in order to focus ion microprobes to small spot sizes, but is also important for the production of proper angular channelling scans u sing beam rocking since such fields distort an otherwise linear scan. In th e absence of sextupole and higher order multipole fields, symmetric pattern s 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. (C) 1999 Elsevier Science B.V. All right s reserved.