Spherical aberration correction with threefold symmetric line currents

It has been shown that N-fold symmetric line current (henceforth denoted as N-SYLC) produces 2N-pole magnetic fields. In this paper, a threefold symmetric line current (N3-SYLC in short) is proposed for correcting 3rd order spherical aberration of round lenses. N3-SYLC can be realized without using magnetic materials, which makes it free of the problems of hysteresis, inhomogeneity and saturation. We investigate theoretically the basic properties of an N3-SYLC configuration which can in principle be realized by simple wires. By optimizing the parameters of a system with beam energy of 5.5keV, the required excitation current for correcting 3rd order spherical aberration coefficient of 400mm is less than 1AT, and the residual higher order aberrations can be kept sufficiently small to obtain beam size of less than 1nm for initial slopes up to 5mrad. •A new magnetic aberration corrector which is free of magnetic materials is proposed.•Correction of 3rd order spherical aberration is confirmed by calculations.•The excitation current for correction can be made less than 1AT for 5.5keV beam.•After correction the beam size is less than 1nm for initial slopes up to 5mrad.