Permanent magnet device for producing axially symmetrical magnetic fields

摘要

504,802. Electron lenses. NICOLL, F. H. Nov. 1, 1937, No. 29818. [Class 39 (i)] [Also in Group XXXV] A permanent magnet device for focusing a composite beam of electrons by means of an axially symmetrical field, comprises two or more coaxial magnetic polepieces spaced longitudinally from each other and having an aperture through which the beam passes, the gap or gaps between the polepieces, which may be adjustable, being straddled by a plurality of permanent magnets arranged symmetrically about the axis of the polepieces with like poles of each magnet associated with the respective polepiece. The magnets may be either of bar form, in which case the polepieces have flanges of magnetic material between which the bars fit, or of horseshoe form, in which case either or both of the polepieces may have flanges serving as end shields. In the forms shown, 1, 2, 2a are soft iron cylindrical cores bridged by horseshoe or bar magnets 6, 6a, the electrons passing near the axis 8. Magnetic flanges 3, 4 may be threaded to allow movement of one of the polepieces for adjustment of the air gap 7. The multi-gap form shown in Fig. 6, may be modified by substituting for the horseshoe magnets bar magnets supported by an additional flange on the core 2a. Numerical data are given.