GB1020633A - Improvements in electron optical system - Google Patents

Abstract

1,020,633. Cathode-ray tubes. GENERAL ELECTRIC CO. Aug. 20, 1962 [Aug. 22, 1961], No. 31883/62. Heading H1D. An electron beam tube comprises an electron gun 10, a prefocusing lens 20, an axially extending spiral accelerating electrode 30, an electrostatic main focusing lens 40 and a target 8, lens 20 providing an image of the beam source for imaging on the target by main lens 40, and the accelerating field produced by electrode 30 being adjustable to vary the effective object distance of main lens 40 to vary the spot size on the target. As shown, the electron beam source includes cathode 11, collimator electrode 12, first anode 13, modulating electrode 14 and miniscus electrode 15, the facing surfaces of electrodes 13, 14 being shaped to form between them a focusing field having hyperbolic equipotentials, whereby a high-density virtual cathode is produced in aperture 18. The prefocusing lens field also has hyperbolic equipotentials, and is formed by electrodes 15, 22 and may be augmented by resistive coatings on support cylinder 24. An accelerating electrode 31 in the form of a spiral resistive coating is connected at one end to electrode 22 and at the other end to a conductive shield coating on the exterior of insulating cylinder 32. This shield coating is also connected to part 41 of a bipotential main focusing lens, the other part 46 of which is connected to envelope coating 48. Alternatively, a unipotential main focusing lens may be used. In operation, the prefocusing lens provides a virtual image of the virtual cathode, and the accelerating electrode 31 has the electrical effect of elongating the effective object distance of the main focusing lens, whereby for the fixed image distance between the main focusing lens and screen 8, the electron beam spot size on screen 8 is correspondingly reduced. Dynamic focusing, as well as control of resolution, is provided by varying the potential of electrode 22. The accelerating spiral 31 may have a pitch progressively increasing towards the target so that it also acts as a moderate converging lens. Electron beam centring at apertures 18, 38 is ensured by coils 60, 67, each comprising two pairs AB, CD, Fig. 3, of coils, each pair being wound in series and supplied with reversible-polarity, adjustable amplitude, direct current as shown. Beam-shaping coils 70 are also provided to enhance spot roundness, or to produce spot-ellipticity at the screen, and may comprise four coils EFGH, Fig. 4 (not shown), wound in series and mounted on an insulating support 71 which is rotatably adjustable about the tube axis. Alternatively, to avoid mechanical rotation of the beam-shaping coils, a further set of four coils, similar to coils EFGH, may be provided, and are displaced 45 degrees relative to the first set, the currents through the two sets of coils each being adjustable from a maximum in one sense, through zero, to a maximum in the opposite sense, with a 90-degree phase difference between the two, Figs. 5 and 6 (not shown).