A double-gap planar magnetic lens

An analytical model and equations for describing the field of a planar magn etic lens with two concentric nonmagnetic gaps are developed. It is demonst rated that the "half-width" of the magnetic field of the lens can be contro lled by varying independently the currents in the excitation coils of the t wo gaps. This makes it possible to control the focal length and the spheric al aberration of the lens. It is shown that when oppositely directed curren ts are used in the excitation coils, a planar magnetic lens that does not r otate the image can be obtained. (C) 1999 American Institute of Physics. [S 1063-7842(99)02608-2].