GB1278827A - Focusing method and device - Google Patents

Abstract

1278827 Automatic focusing control SOC ALSACIENNE DE CONSTRUCTIONS ATOMIQUES DE TELECOMMUNICATIONS ET D'ELECTRONIQUE 14 Aug 1969 [14 Aug 1968] 40685/69 Heading G3R A primary electron beam is maintained focused on a metallic workpiece by collecting secondary radiation, converting the collected radiation into a current, and servo-controlling the beam focusing device to minimize the current. In the embodiment shown in Fig.5, electrons from a source 33 are directed on to a workpiece 41 and secondary radiation is detected by an annular plate 27 which is connected through a servo-control device 28 to the main beam focusing coils 31. The secondary radiation consists of three components, viz. high-energy electrons, low-energy electrons and positive ions. While it is stated that all these components have a flux minimum when the beam is correctly focused on the workpiece, the minimum in the high-energy electron flux is the simplest to work with since it is not complicated by adjacent maxima, Figs.6a- 6c (not shown). For this reason, the other components are removed from the secondary beam by appropriately biased conical elements 35, 38. In operation, the system may be used for seam welding, and the length of the beam path may change as the workpiece 41 is moved. The arrangement then acts to adjust the focusing coils 31 and maintain the beam correctly focused. In a further embodiment, Fig.11 (not shown), two systems as shown in Fig.5 are used to control the melting of a metal bar and the presence of a pool of molten metal in a mould. The electron beam is focused partly on the bar and partly on the pool, and the systems operate to ensure a satisfactory ratio between the energies received in each case.