Patent GB-853061-A (GB)
Title: Improvements in or relating to periodic magnetic focusing systems for electron beam tubes
IPC: H01J23/087
Filed: 1957-02-26
Abstract: 853,061. Travelling wave tubes. SIEMENS & HALSKE A.G. Feb. 26, 1957 [March 1, 1956], No. 6467/57. Class 39(1). A periodic magnetic focusing system for a travelling wave tube comprises a plurality of pole pieces spaced apart and defining a longitudinal axis (coincident with the electron beam axis) and a plurality of permanent magnets, the magnetic axis of each of which lies in a plane perpendicular to the said longitudinal axis. The magnetic field preferably has a sinusoidal distribution in space along the electron beam. In one form, Fig. 3, there are a large plurality of pairs of permanent magnet bars 13, 14 ; 9, 10, connected by pole-pieces 11, 15 having central apertures for the travelling wave tube, each successive pair of magnets being rotated by 90 degrees with respect to the proceeding pair and the central poles 15 being alternately north and south, each pair of magnets having inner adjacent ends of the same polarity. The outer ends of the magnets are connected by a square casing 16, 17 with flattened comers of thick soft sheet iron of negligible reluctance. In a modification, Fig. 6, there are four longitudinally arranged bar magnets 18, 19, 26, 27 magnetised in a radial direction and connected at intervals by pole pieces having plane portions 22 and tapered portions 21, 24, the tapered portions being alternately connected to the horizontal and vertical pairs of magnets, and the assembly being surrounded by a casing of thin soft sheet iron as before. The thickness of the central pole-pieces should bear a definite relationship to the lengths of the gaps between them. The horizontal and vertical pairs of magnets may be replaced by two U-shaped magnets. In another modification, Fig. 8, four magnets 45, 46, 47, 48 are arranged to define the sides of a square, pole-pieces 41, 42, 43, 44 being arranged at the comers. To provide a uniform magnetic focusing field equal to the RMS value of the periodic field across the input and/or output feeders an arrangement comprising two pairs of bar magnets 49, 50, 51, 52, Fig. 9A, connected by soft sheet iron yokes 53, 54 and annular pole-piece 56 and soft iron sheets 34, 38 contacting magnets 18, 19 being placed at the respective inner ends of the two of magnet bars, is placed at the end of the periodic focusing system. The invention can also be applied to periodic magnetic focusing assemblies with 3, 4 or 6 permanent bar magnets replacing each pair. The pole-pieces are in all cases of soft iron. The arrangement of Fig. 9A can be modified to allow the localised feeder focusing arrangement to interrupt rather than to terminate the periodic focusing system. The travelling wave tube may be an amplifier or oscillator and uses a helix delay line. A prior art magnetic periodic focusing arrangement is described with reference to Fig. 1, in which short annular permanent magnets are spaced by soft iron rings having tubular portions at their inner ends to serve as pole-pieces, and a unidirectional (not periodic) magnet arrangement having bar magnets arranged along the beam at angles of 60, 45 or 30 degrees respectively to one another. Soft iron cylinders connecting adjacent magnetic poles to make the field more uniform are referred to. Reference has been directed by the Comptroller to Specification 757,369.