GB426087A - Improvements in or relating to cathode ray tube and like electron beam apparatus - Google Patents

Abstract

426,087. Cathode-ray tubes. TELEFUNKEN GES. FUR DRAHTLOSE TELEGRAPHIE, 12, Hallesches Ufer, Berlin. Feb. 6, 1934, Nos. 3936 and 3937. Convention dates, Feb. 6, 1933 and July 31, 1933. [Class 39 (i)] An electron lens comprises two or more elongated conductive hollow bodies coaxial with respect to the electron beam axis and means for applying potentials to the bodies, the potentials being such that the potential gradient is reversed at some point along the path of the beam, and the apertures in the bodies being so large that only the lines of force commencing on the internal walls of the bodies produce a lens effect. The bodies may be of circular, elliptical or rectangular cross-section. In one form, Fig. 4, an anode consists of a diaphragm 5 and a net or mesh cylinder 6 which surrounds a cylinder 7, the members 6 and 7 constituting the electron lens. The remainder of the tube other than the screen 2 is coated with a silver layer 9 connected to the anode. In a modification, the cylinder 6 is replaced by a coating on the wall, and the cylinder 7 by another coating between the coatings 6 and 9 and maintained at a negative potential relative to them. The coatings may be replaced by tubes or nets, or one or more of them may form part of the tube envelope. The negatively biassed cylinder may have the modulating or cutting - off potential applied to it, the potential of the Wehnelt cylinder then remaining constant. In another modification two short bodies may be arranged inside a long body. Fig. 6 shows an arrangement which prevents " chromatic " aberration due to the presence of rays of different hardness and " spherical " aberration due to the large cross-section of the rays. The hollow bodies 10... 19 are connected to tappings on a potentiometer 20 which has a positive pole connected to its mid-point and a negative pole to its ends. If a divergent lens is required these potentials are reversed. In a modification the elements 10 ... 19 are replaced by the turns of a resistance helix furnished with suitable tappings.