GB1143311A - Improvements in or relating to electron beam magnetic focussing systems - Google Patents

Abstract

1,143,311. Electron beam focusing apparatus. SIEMENS A.G. 25 Jan., 1966 [26 Jan., 1965], No. 3225/66. Heading H1D. A magnetic system for focusing an electron beam along an axis-e.g. for a travelling wave tube-includes a plurality of permanent magnets magnetized in directions perpendicular to the axis and disposed in a closed row or rows along and symmetrically about the axis with similar poles adjacent the axis, the poles remote from the axis being linked by a magnetically soft bridging member which extends beyond the magnet at each end of the system to each of two magnetically soft irises, the individual magnets producing fields along the axis which are larger the nearer the magnet is to the centre of a row, and the field along the axis of the complete system reversing at the centre of the row or rows. A periodic focusing arrangement, Fig. 4, includes two or more axially adjacent systems defined by successive pairs of irises having a common pair of bridging members 2 from which an opposed pair of sets of hard ferrite magnets extend, the lengths of the magnets varying in a step-wise manner to a maximum at the centre of each system, such as to provide an axial field of substantially uniform intensity which reverses sense at each centre plane, V, and in the plane of each iris. In alternative embodiments, four sets of symmetrically disposed magnets are arranged around the axis (Fig. 2, not shown), or a single set constituted of annular disc magnets is used. Reference is also made to the production of non-uniform reversing fields by suitable variation of the magnet steps.