GB1312048A - Electron discharge devices - Google Patents

Abstract

1312048 Velocity modulated tubes EMI Ltd 3 July 1970 [18 July 1969] 36398/69 Heading H1D An electrostatic focusing system-e.g. for a travelling-wave tube or a klystron-includes a member apertured for the passage therethrough of the electron beam, which member has a charge-retaining and secondary electron emissive surface surrounding the aperture, such that, in operation, unfocused electrons of the beam impinging on this surface charge it to the correct potential required for focusing the beam, thereby dispensing with the need for external leads to the focusing system. As shown, in a klystron, Fig. 1, the apertured members 16, 17, 18 may be mounted in ceramic insulators 19 (Fig. 2, not shown) between the walls 7, 8 of successive cavities, with which walls they form, in operation, electrostatic lenses. The members may be of insulating material, in the form of annuli, or as shown in Fig. 8, in which the internally projecting portion 62 defines the aperture and is coated with secondary emissive material, the leakage path to the conductive portions of the discharge tube being increased by the provision of annular or spiral grooves 64, ... ; 79. The coating may have a secondary emission coefficient less than unity-e.g. soot, tantalum carbide, or titanium dioxide, in which case the member stabilizes at cathode potential, or greater than unity-e.g. copper-when it stabilizes at the second cross-over potential, the focusing system in the two cases being designed for operation at those respective potentials. When the layer is of non-conductive material, it may be provided with a conductive underlayer, to ensure a uniform distribution of potential.