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 Accelerato r coupled to a Scanning Proton Microprobe (SPMP) beamline. The finite element method is used to calculate the electrostatic field, and optic al properties are extracted from my tracing. Aberrations for the two e lement lens are found to be unacceptably high except for prohibitively large accelerating voltages. For three element lenses, both accelerat ing and decelerating modes are analysed with a range of final to initi al voltage ratios of between 1 and 8. It is found that, although aberr ations are lowest for accelerating modes, the high voltage nature of t he 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 car ried out using a minimization routine. Decelerating mode aberrations h owever are not dramatically reduced, but it is suggested that miniatur ized versions of the decelerating lenses are a practical solution, pro viding improved SPMP resolution.