OPTIMAL NONLINEAR, ORTHOMORPHIC, MAGNETIC QUADRUPOLE LENS SYSTEMS FOR ION MICROPROBES

The lens strengths, demagnifications and spherical aberrations of many different quadrupole focusing systems for an ion microprobe are calculated and compared. The paper studies triplet and Russian quadruplet combinations of magnetic quadrupoles which give equal (in magnitude) demagnification in the two planes of focusing. The matrizant (or the transfer matrix) of the third order is determined for different geometries of the studied systems. Numerical optimization of the quadrupole systems is applied. The above matrizant for given geometries is used and the size of the object (first) and aperture (second) diaphragms is varied to obtain the minimum spot size at the specimen for a given beam emittance. The optimal systems are discussed.