AN INVESTIGATION OF LOW ABERRATION ELECTROSTATIC LENSES FOR USE WITH A FIELD-IONIZATION ION-SOURCE AND A PROTON MICROPROBE

Two and three element electrostatic lenses are analysed for use with a high brightness field ionization ion source in a Pelletron Accelerator coupled to a Scanning Proton Microprobe (SPMP) beamline. The finite element method is used to calculate the electrostatic field, and optical properties are extracted from my tracing. Aberrations for the two element lens are found to be unacceptably high except for prohibitively large accelerating voltages. For three element lenses, both accelerating and decelerating modes are analysed with a range of final to initial voltage ratios of between 1 and 8. It is found that, although aberrations are lowest for accelerating modes, the high voltage nature of the field ionization source would almost certainly prelude their use in practice. Alternative, decelerating, modes are investigated for these lenses used as Einzel lenses, and electrode shape optimization is carried out using a minimization routine. Decelerating mode aberrations however are not dramatically reduced, but it is suggested that miniaturized versions of the decelerating lenses are a practical solution, providing improved SPMP resolution.