Patent GB-715262-A (GB)
Title: Improvements relating to electron discharge tube arrangements
Inventor: HOSKING WILLIAM JAMES CECIL
IPC: H01J31/60
Filed: 1950-06-09
Abstract: 715,262. Electron discharge apparatus. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 11, 1951 [June 9, 1950], No. 14485/50. Class 39(1) [Also in Group XL(c)] In an electron discharge tube arrangement comprising an electron discharge tube having a charge storage target and a cathode for generating electrons, which can scan the target to generate signals from the stored charges, the target is stabilized at cathode potential and signals to be stored on the target are applied to it from the cathode so that charged areas assume a negative potential with respect to the equilibrium potential of the target. In the arrangement shown in Fig. 1 the target 5 comprises spaced particles of insulating material on a metal plate from which an ouput is taken to a display cathode ray tube 14. With the target about 3 volts positive to the cathode 2 the insulating areas are stabilized at cathode potential and the signals to be stored are then applied to the cathode in a negative sense and are also applied to the electrodes of the tube to prevent defocusing; the target 5 is charged to potentials varying from the original cathode potential to a more negative potential corresponding to a maximum signal amplitude. To reproduce the signal the target is scanned and electrons will penetrate to the signal plate when the insulating elements are at cathode potential and not where the insulating elements have their maximum negative charge; the current depends on the charge and gradation is present in the complete optical image. To discharge the cathode may be brought to a negative potential of about 100 volts so that it generates secondary emission at the target or the mesh 10, which normally acts as an ion trap, may be brought to a negative potential of 30 volts relative to the wall anode to allow positive ions to discharge the target. In another arrangement, Fig. 2, the signals to be stored are displayed on a cathode ray tube 21 and reflected by a half-silvered mirror 20 on to the photo-cathode 17, the emission of which charges the target 5; the cathode 17 is pulsed to a negative potential during charging. To scan the charges a shutter 22 is closed to prevent further charging and a scanning light shot from a cathode ray tube 81 focused on the photo-cathode; preferably the shutter 22 is so arranged that charging takes place during the fly-back period of the scanning beam on the tube 18. The tube 16.may be provided with a positive mesh 10 to reduce electron transit time and increase definition, the target may be discharged by raising the mesh 10 to a high potential or reducing the cathode to a negative potential of about 100 volts. The target 5 is preferably formed by evaporating calcium fluoride on a grid having 600 or more meshes per linear inch, the grid being mounted with its uncoated surface in contact with a metal plate provided on an insulating base. Alternatively willemite, calcium fluoride or magnesium fluoride particles may be settled on a metal plate, or the surface of an aluminium plate may be anodized at appropriate places.