GB941734A - Improvements in electron gun - Google Patents

Abstract

941,734. Cathode-ray tubes. GENERAL ELECTRIC CO. March 20, 1961 [March 21, 1960 No. 10042/61. Heading H1D. An electron gun has a relatively large area cathode 20, Fig. 4, and a decelerating longitudinally non- symmetric electrostatic hyperbolic lens 24, 26, which both modulates the beam, by electron reflection, and focuses it at a small spotdefining aperture 46 in exit electrode 28. The focusing of the beam cross-over at the defining aperture occurs at the same modulationvoltage as provides maximum current transfer through the modulating or " gate " electrode 26. The gate electrode 26 is cut away, or is formed from sheet material, to provide a conical surface 40 having an apex angle of 109 degrees, the apex of the surface coinciding with the defining aperture 46. Exit electrode 28 acts like an optical meniscus lens and has a similar conical surface 42 in back-toback relationship with surface 40. Anode 24 has a spherical surface 48, all these surfaces 40, 42 and 48 approximating to rheoretical hyperbolic surfaces. The cathode 20 and electrode 22 form a beam collimating system of the Pierce type. The beam emerging from the aperture 46 is reconverged by the lens 28, 30. In a modification the beam is reconverged by a symmetrical hyperbolic unipotential lens. In Fig. 5, the gun system is accommodated in a container 150 of which the apertured electrode 128 forms an end portion. The container 150 may be separately evacuated to a higher vacuum than that obtaining in the main portion of the apparatus to which the container is connected. In this arrangement the cathode may be of the oxide-coated type.