The electron optical properties of magnetic lenses are determined by the ratio S/D of gap width S to bore diameter D, by the actual size of the gap and by the lens strength NJ√U*. Objective lenses of particularly small aberrations are obtained if the specimen is positioned close to the center of the gap and if the width of the gap is made as small as possible. For projector lenses the best performance results if the lens is employed at its minimum focal length. Another aspect important for high resolution lenses is that a stigmator is available for the correction of axial astigmatism. With ferromagnetic super-conducting lenses, both permendure and rare earth metals (these cooled to liquid helium temperatures) are employed for the pole pieces in combination with an iron casing which enshrouds the superconducting coil. With the superconducting shielding lens, the superconducting coil is encapsulated by a superconducting shield which has about the same general shape as the iron circuit casing of a normal lens, whereby the narrow gap in the shielding tube around the axis allows the magnetic field there to penetrate into the space close to the axis and form the electron lens.
Riecke, Wolfgang Dieter
"Magnetic Electron Lenses,"
Scanning Electron Microscopy: Vol. 3
, Article 1.
Available at: https://digitalcommons.usu.edu/electron/vol3/iss1/1