Patent GB-815836-A (GB)
Title: Improvements in or relating to electron discharge devices including magnetic focussing arrangements
IPC: H01J23/00
Filed: 1956-10-23
Abstract: 815,836. Magnets. WESTERN ELECTRIC CO. Inc. Oct. 23, 1956 [Oct. 27, 1955], No. 32265/56. Class 35. [Also in Group XL (a)] In a travelling-wave tube apparatus, the electron beam is focused by a permanent magnet having a central tunnel within which is a flux guide consisting of two plates on either side of the delay line, or preferably a cylinder surrounding the delay line. The permanent magnet may be specially shaped, e.g. by making it at least partly elliptical or ellipsoidal, or the flux guide may be specially shaped, e.g. by decreasing its thickness from both ends towards the centre, so as to make the magnetic field axial, parallel, and uniform. The magnetic field extends over the entire length of the delay line. In one form, Figs. 1, 2, the permanent magnet 10 comprises a central portion 11 with elliptical side walls and a tunnel 12 for the electron beam and helix, and having flat top and bottom walls abutting upper and lower sections 13, 14 which project at the sides and ends, and has elliptically shaped ends 15 to maintain the field uniformity in the end spaces which house the waveguide feeders. The whole magnet of Alnico V or VI producing a field of 6000 gauss can be a single integral unit or can be in two equal sections divided by a plane through the major axis of the main ellipse, or each of the centre upper and lower sections can be separate members secured together. The flux guide 25, which may be a thin-walled seamless iron tube, has rectangular polepieces 27, and further polepieces 28 are arranged at the ends of the upper and lower magnet sections. The flux guide can be used to adjust the linearity of the field by providing the magnet 10 with a framework 31 having adjustment screws 32, 33, 34, screws 32, 33 (screws 33 via spring fingers 38) varying the position of polepieces 27 in two transverse directions at right-angles to one another. Screws 34 vary the transverse position of polepieces 28. Undesired transverse magnetic field components due to inhomogeneities in the magnetic material and to the closeness of the magnetic material to the axis can be compensated by (a) the shielding effect of the flux guide tube 25; (b) coaxial rings of magnetic material supported on a non-magnetic tube surrounding the flux guide 25 and spaced apart from one another by non-magnetic rings, Fig. 3 (not shown), or a single tube of magnetic material with suitable non-magnetic supports; (c) (used with or without (b)) by high u discs, fixed to and extending inwardly from the walls of the flux guide, Fig. 4 (not shown). Flux guides 30, preferably rectangular plates, may be used in the end spaces or bridging areas for field uniformity and adjustment, particularly when the bridging top and bottom magnet parts do not have elliptical ends as in the embodiment of Figs. 5, 6 (not shown). In Figs. 5, 6 (not shown), a single unit permanent magnet of rectangular cross-section contains in the cylindrical tunnel thereof a cylindrical flux guide whose thickness varies linearly from a minimum at the centre to a maximum at the ends to give a uniform rectilinear field inside it and flux guide plates similarly tapered in the end or bridging spaces of rectangular cross-section. In another form, Fig. 7, the magnet is wholly ellipsoidal apart from the cut away end portions and no special shaping of the flux guide is necessary. The end of tunnel 203 of square or rectangular section assist in coupling to waveguide feeders, and flat flux guide plates are positioned inside the tunnel 203, the flux guide plates extending also over the full thickness of the polepieces at each end of the magnet. A single cylindrical flux guide of uniform wall thickness extends from end to end of the magnet structure, Fig. 8 (not shown). The flux guide can be moved transversely for field rectilinearity adjustments by screws acting on the polepieces at the ends of the magnet. The input and output feeders can be coaxial lines, Fig. 8 (not shown). Either or both of the shielding devices of Figs. 3, 4 (not shown) can be used in Figs. 5, 6, 7 or 8 (Fig. 7 only shown). Specification 726,823 is referred to.