GB798257A - Improvements in or relating to electron discharge devices of the hard type and to electrostatic lenses therefor - Google Patents

Abstract

798,257. Cathode ray tubes. TELEFUNKEN GES. FUR DRAHTLOSE TELEGRAPHIE. Nov. 15, 1954 [Nov. 13, 1953], No. 32959/54. Class 39(1). A hard cathode ray tube has an electron lens serving to form the sole focusing means for focusing the crossover adjacent to the cathode to a spot at the screen, the lens consisting of a tubular electrode 5, Fig. 2, and a further electrode 6 biased to form an acceleration lens, the distance between the cathode and the effective end of lens electrode 5 remote from the cathode is less than the internal diameter of the lens electrode 5 so that an optimum spot size is obtained at the target with a smaller potential on the lens electrode 5 than if the above dimensional relationship is not fulfilled. In a modification the desired field is simulated by a construction comprising a layer of resistive material 1, Fig. 3, in combination with conductive rings 10 of progressively decreasing axial dimensions one end of the resistive layer being connected to the second lens electrode 11 and the other end being held at a potential -100 to +500 volts different from the potential of 11 so as to set up a potential gradient along electrode 9, 10. This modified embodiment is used when the diameter of the neck is too small for the lens electrode 9, 10 to have a greater diameter than the distance of its remote end from the cathode, and is useful for tubes having ion traps. If the second lens electrode 6, Fig. 2, extends within the first lens electrode, as shown, the effective end for the dimensional relationship above. referred to is the part of the first lens electrode registering with the end of the second lens electrode while if the second lens electrode is wholly outside the first lens electrode the effective end is the real end. The first lens electrode is at -100 to + 500 volts and the second lens electrode positive thereto. The second lens electrode 6 can be supported from the first by ceramic disc 7. Frusto-conical member 8 screens disc 7 from the lens field and also reduces lens aberrations caused by the aperture at the waist of 6. If conical member 8 is dispensed with, disc 7 is coated with conductive material connected to 6 to enable the magnetic scanning field to be close to the lens field, lens electrodes 5, 6 are made of material of poor electrical conductivity such as non-magnetic steel or bronze so as to minimize eddy current losses. The lens field can be magnetically screened by making lens electrode 5 of ferromagnetic material. Electrode 5 can alternatively be a conductive coating on an insulating tube fitting closely within the tube neck or a conductive coating on the tube wall. Both surfaces of disc 7 can have annular ridges to increase the leakage path.