GB846897A - Magnetic electron lenses - Google Patents

Abstract

846,897. Electron lenses. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 17, 1957 [Oct. 25, 1956], No. 32584/56. Class 39(1). A magnetic lens for collimating rays emanating from a point O, Fig. 1, consists of a coil 4 wound uniformly on a magnetic member 1 to generate in the space between it and a second magnetic member 2 a leakage field which is zero at the centre and increases linearly, in opposite sign, towards either end of the space. Distortion of the field at the ends of the coil may be connected by additional coils wound around the non-magnetic spacers 3. In Fig. 3 the lens comprises two pairs of tapered pole pieces 6, 6 and 7, 7 energised by coils 9, 9 wound on the yokes 8, 8. The pole pieces may be energised instead by permanent magnets or, preferably, by yokes permanently magnetised to provide half the magnetising field required in combination with coils supplying the other half of the magnetising field. A magnetic screen 10 with a convex front may be provided and to compensate for the presence of the screen, the pole pieces 7 are slightly less tapered than the pole pieces 6 and are also slightly longer. The pole pieces may also be made to bulge in width between the yoke and the tip to avoid cylindrical aberration. The tips of the pole pieces may be bent outwards to secure a better field distribution and the pole pieces may be tapered in the horizontal plane to be convergent in the direction of the lens axis so as to reduce divergent action and to avoid sudden transition into the influence of the lens. The lens axis need not be central. The lenses may be used in the " flat " cathode ray tube of Specifications 739,496 and 794,257. In Fig. 4 the ends of the pole pieces are shown shaped to fit the tube envelope 12 and the yokes 13 and energising coils 9 are outside the envelope.