Fifth-order aperture aberration of electrostatic quadrupole lens systems

The fifth order aperture aberration is investigated in three types of quadrupole lens systems: the conventional regular antisymmetric quadruplet, the quadruplet made of combined quadrupole-octupole systems in which the third order aperture aberration is eliminated, and the mid- acceleration quadruplet produced by applying an accelerating voltage to the two middle lenses that results in the essential reduction of the aperture aberration. It is shown that for small initial trajectory slopes (less than 0.02) the quadrupole-octupole system provides smaller image blurring. For higher trajectory slopes when the fifth order aperture aberration becomes dominant the smallest beam spot is formed by the mid-acceleration quadruplet.