Patent GB-757369-A (GB)
Title: Improvements in or relating to electron beam focusing systems
IPC: H01J23/087
Filed: 1954-04-28
Abstract: 757,369. Electron-beam apparatus. WESTERN ELECTRIC CO., Inc. April 28, 1954 [April 29, 1953 ; April 29, 1953 ; April 29, 1953; April 29, 1953], No. 12311/54. Class 39 (1). [Also in Group XXXV] An electron beam interacting with a wave transmission circuit is focused by a periodically varying magnetic field such that the longitudinal magnetic field in successive regions along the beam reverses in direction. It is shown mathematically that a beam of oscillating crosssection is produced which will remain stable within limits provided the r.m.s. value of the longitudinal field is of the same order as the uniform field which would produce Brillouin focusing; diagrams are given of the stable and unstable regions of the Mathieu functions involved. The resultant concentration of the field gives a reduction of total stored energy and thus of magnet weight over the uniform field method. Permanent magnets or electromagnets may be employed. In a travelling- wave tube, the helix 26, Fig. 2, is surrounded by a number of pole-pieces 38 of any high permeability material, including ferrites, connected by alternately opposed horse-shoe magnets 36. The input and output waveguides 31, 32 have their opposed walls formed as polepieces bridged by magnets 37. A further lens surrounds the anode 24 of the electron gun 22, 23, 24. The pole-pieces may be discs joined by bar magnets or may themselves form components of a corrugated delay line. A number of axially polarized disc magnets may be spaced along the outside of the envelope. In another form the magnets 61, Fig. 6, are cylinders with an internal toroidal cavity 62 containing a few turns of wire 64 for initial magnetization. A succession of solenoids 133, Fig. 13, may co-operate with armatures 134 to produce the desired field when oppositely connected. The input and output waveguides may be similarly shaped and energized by electromagnets 137, 138; and an initial focusing coil 143 is provided. The analysis shown to apply to cases where a hollow beam passes between the inner and outer conductors of a coaxial line, Fig. 14 (not shown), or outside (Fig. 15), or inside, Fig. 16 (not shown), a delay line, provided that in each case a radial electrostatic field is set up to simulate the space charge field of a solid beam. This may be done by applying a potential difference between the conductors of the coaxial line or by providing a separate field conductor such as 179, Fig. 15, which here is outside the line 175 but may be a rod inside it. A converging hollow beam is produced by annular cathode 172 and gun system 173. This arrangement permits direct axial input and output by lines 177, 178 and 182, 183, respectively. The line 175 is a grooved rod with impedance matching spacing of the grooves at the ends. An imbricated pole arrangement for producing the successive opposed fields by means of only four long magnets 221 ... 224, Figs. 18, 19, comprises a number of pole-pieces 229A, 229B, alternately bridging magnets 221, 223 and 222, 224. The field lines spread from a given pole-piece in opposite directions to the adjacent pole-pieces and back through a return circuit provided by rectangular casing 225 ... 228. Horseshoe magnets 235, 236 provide initial and final fields. A thin-walled magnetic cylinder 232 serves to smooth out the quadrupole components of the field. In a modification, Fig. 21 (not shown), only one diametral pair of magnets, such as 221, 223, is provided and the pole. pieces are connected alternately to these. The pole-pieces may form components of an interdigitated delay line. Specifications 669,474 and 726,823 are referred to.