Numerical synthesis of an optimal microprobe focusing system

An electrostatic microprobe focusing system including a separated Russian q uadruplet and two slits (or diaphragms) is numerically synthesized, taking into consideration all geometrical aberrations of the third order. The matr izant method is applied to minimize the beam spot size for a fixed beam cur rent. For quadrupoles consisting of four copper rods, the electric field in side of the quadruplet is computed using an accurate version of the boundar y charge method. The minimum spot size, the appropriate radii of the diaphr agms, the distance between them, the effective length and the polarisation for different diameters of the rods are found. (C) 1999 Elsevier Science B. V. All rights reserved.