The chromatic and spherical aberration of round electron optical lenses with spatially periodic electrostatic fields: cone-edge and cone focusing

Abstract

Whereas conventional foilless electrostatic lenses without axial electrodes or space charge show positive spherical Cs and chromatic Cc aberration for paraxial rays, results presented here show that for periodic or quasiperiodic electrostatic fields, low or negative values of Cc and Cs can occur if the rays pass through zonal regions. Not only can cone-edge focusing occur, but cone focusing can be found at several values of the emission angle. As a possible application, charged particles of sufficiently high velocity originating from a weak point source, for instance, Auger or internal conversion electrons, can be focused onto a small spot by utilizing an annular rather than a circular aperture.