Low-aberration optics for large-angle beam with unit core lenses

The characteristics of core lenses are numerically examined for a large-angle beam whose aberration is huge for a circular-lens system. The unit core lens, which both, focuses and deflects, is defined by ring charges. The geometric and energy aberrations are analyzed using the diagrams of axis-intercept. For a single unit core lens or two cascaded unit core lenses, the aberrations cannot be commonly reduced over a wide range of initial beam conditions. By cascading three unit core lenses, both geometric and energy aberrations are small for a symmetric-trajectory beam over a wider range of initial beam conditions. In addition, a procedure for searching suitable conditions is explained because the number of setting parameters is 12. An example shows that taking diffraction aberration into account, the resulting beam diameter is 4.7 nm for 10 keV beam energy.