Patent GB-772091-A (GB)
Title: Improvements in or relating to electron tube arrangements consisting of a travelling-wave tube and a magnetic focusing system
IPC: H01J23/087
Filed: 1954-09-14
Abstract: 772,091. Magnets. SIEMENS & HALSKE AKT.-GES. Sept. 14, 1954 [Nov. 13. 1953], No. 26665/54. Class 35. [Also in Group XL (a)] A travelling wave tube has a magnetic focusing device comprising a plurality of closed ferromagnetic circuits with a common flux guide of soft iron, each circuit having at least one permanent bar magnet, the magnetic circuits being completed by soft iron members, the magnetic fields all being in the same direction and producing an axial magnetic field in the tube for focusing at least one electron stream as a parallel sided beam throughout the length of the flux guide. The beam may be cylindrical or annular and there may be one beam or two beams travelling in the same direction or in opposite directions. In one embodiment, Fig. 1 (not shown), four permanent magnets are spaced around a soft iron tube, soft iron end plates completing the magnetic circuit. The magnet system can support the discharge tube, and the tube leads, especially high-frequency leads, can be secured to the magnet system directly or indirectly. If the magnetic field is not to reverse at the end, a soft iron collar can be secured to one of the end plates, Fig. 3 (not shown.). Alternatively, if field reversal over a short length is desired, a magnetic screen (such as 39, Fig. 9) is provided adjacent one of the soft iron end plates but has opposite polarity with respect thereto, Fig. 4 (not shown). The arrangements of Figs. 3, 4 can be applied at both ends of the magnet assembly. In another form, Fig. 5A, two sets of four permanent magnets 10-13, 14-17 are secured to the flat outer faces of the thickened ends of a soft iron tube 7 and the magnetic circuit is completed by four soft iron bars 18- 21. In another form, Fig. 6, there are four sets of radially extending permanent magnets 10, 12; 14, 16; 26, 28; 27, 29 (only two of each set of four shown) connected by bars 22, 24; 23, 25; 30, 31. Members 30, 31 are preferably replaced by a soft iron tube. Alternatively, there may be a permanent magnet between each pair of connecting bars 22, 24; 23, 25 instead of permanent magnets 26, 27, 28, 29 and members 30, 31. In one such form, Fig. 7 (not shown), tube 7 is in two parts of different diameters, the part of larger diameter being so thick that there is no field. inside it so that it can house the gun of a travelling wave tube. This part also has apertures for hollow tubular high-frequency conductors. In another form, Fig. 9, there are six sets of four permanent magnets 10, 12, 14, 16; 43,47; 44,48; 45, 49; 46, 50; 14, 16, soft iron bars 33, 34, 35, 36 and soft iron cylinders 37, 38. Screen 39 can be moved axially by sliding members 40 to adjust the strength of the negative field. The soft iron tube 7 is divided into two concentric tubes 41, 42. Instead of four external magnetic paths there can be two or more than four. The beam may be made to diverge as it approaches the collector by means similar to that of Fig. 7 for the gun end. Alternatively, the magnet system may terminate at the output end of the delay line so that the beam after emerging from the magnet system is focused to a lesser extent in the region of opposite field and then diverges to the collector. The soft iron tube 7 may be made up of a number of elements, e.g. bars. The cross-section or permeability of plates is greater than that of tube 7.