GB864272A - Electron gun - Google Patents

Abstract

864,272. Electron beam apparatus. RADIO CORPORATION OF AMERICA. June 25, 1958 [July 22, 1957], No; 20454/58. Class 39(1). An electron gun in a cathode-ray tube comprises an annular cathode 22 coaxial with the tube and having an electron-emissive inner surface, a probe electrode 46 centrally disposed with respect to the cathode and having an end portion 48 shaped to direct electrons in the direction of the tube axis and toward the target electrode and a plurality of annular electrodes 32, 34, 37, 39 coaxially arranged between the probe electrode and the cathode to provide an accelerating field therebetween. As shown, during tube operation the control grids 32, 34 are connected together and biased negatively with respect to the cathode while the accelerating grids 37, 39 are at the same or different positive potentials, the probe electrode being at cathode potential or slightly negative thereto so that electrons from the cathode are focussed and accelerated first radially and then axially on to an aperture 54 in object electrode 50. Electrons passing through aperture 54 are focussed and accelerated by electrodes 56, 64, 66, 68 to converge to a small spot on the fluorescent screen, the spot-size being practically independent of beam current. Examples of electrode dimensions and operating potentials are given. The indirectly-heated cathode 22 is formed as an annular ring having a U-shaped cross-section and is fixed to a mounting ring 24 by a plurality of metal straps 26 each having one end fixed to the cathode and the other to ring 24 so that any expansion of the cathode merely rotates it about its axis without altering its axial position. The ring 24 and the remaining electrodes are supported by glass rods 30. The field configuration in the electron gun is such that only those electrons emitted perpendicularly from the cathode surface are able to pass through aperture 54 thus eliminating fringe electrons which would normally result in distortion of the beam spot. Any positive ions generated by the beam which pass through aperture 54 are attracted by the probe electrode and do not bombard the cathode. Instead of operating electrode 39 at a potential higher than that of electrode 37 in order to obtain the desired field configuration, the cylindrical wall portion 42 may be made thicker than wall portion 40 or may be of smaller diameter, the electrodes then being operated at the same potential.