GB1461415A - Electron guns - Google Patents

Abstract

1461415 Electron beam furnaces GENERAL ELECTRIC CO Ltd 9 Dec 1974 [12 Dec 1973] 57568/73 Heading H1D A glow discharge electron gun, e.g. for electron beam heating apparatus, having an anode structure with a generally planar, apertured first surface facing the cathode 13 and spaced from it a distance less than the minumum at which a gaseous discharge can occur between them, and a second surface 16 defining, on the side of the aperture remote from the cathode, a volume through which the beam passes in operation, having a cross-sectional shape, at least at its end adjacent the aperture, corresponding to that of the aperture, and extending along the beam path for a distance not less than the minimum width of the aperture, which is preferably not less than 8 mm., the dimensions of the aperture and volume being such that, for a range of anode to cathode operating voltages the boundary of the plasma 26 is crossed by the electron beam within the volume, whereby variation of focusing with beam current is minimized. As described, the first and second anode surfaces are provided by different parts 15 and 16 of the same anode which is supported by a metal tube 14 surrounding the aluminium rod cathode 12, the metal supporting plates 7 and 9 being insulated from one another by insulators 6 and 8, to which they are sealed by 0-rings 11. The chamber 3 is continuously evacuated via an aperture in the support tube 2 for the work-table 1, and argon is admitted via the port 10 in metal wall 4 to maintain a pressure of 0À1 torr. Tube 16 may have an internal diameter of 15 mms. and a length of 30 mms., the beam current arising both from secondary emission at the cathode due to ion bombardment, and also from the hollow cathode effect produced when the anode is maintained negative with respect to the worktable 1; the latter effect produces partial compensation for the charge of beam current with gas pressure arising from change of temperature during operation. The anode structure may comprise more than one anode, permitting the first and second surfaces to be held at different potentials, and the beam may be of pencil, annular, or rectangular shape. It is stated that a desired variation of focus with beam current may be achieved by suitably varying the crosssection of tube 16 along its length.