Patent GB-880197-A (GB)
Title: Improvements in or relating to travelling-wave tubes
IPC: H01J23/24
Filed: 1959-06-03
Abstract: 880,197. Travelling wave tubes. SIEMENS & HALSKE A. G. June 3, 1959 [June 3, 1958], No. 18977/59. Class 39 (1). A travelling wave tube has a slow wave structure including a hollow electrically conductive waveguide traversed by an axial electron beam conductive regularly speced baffles being arranged along the interior of the waveguide and are integral with or in direct conductive contact with the waveguide wall, each of the baffles extends over only a part of the interior cross sectional area of the waveguide, the cross sectional area of each baffle is not less than of the cross sectional area of the interior of the waveguide, wherein at least the major portion of adjacent baffles lie on opposite sides of a first plane containing the axis of the waveguide, wherein the disposition of the baffles is such that each is bisected by a second plane containing the axis and wherein the dimensions of the slow wave structure are such that the fundamental is a backward wave. The slow wave structure is interdigitated. The object is to obtain a wide band width and a good coupling resistance by suitably shaping and dimensioning the baffles. The tube may be an amplifier or oscillator. The baffles a, c Fig. 1 have crosspieces on one side and are annuli on the other side and are of the same shape so that they can be produced by the same stamping tool and annular baffles b are produced by another stamping tool. Baffles a and c are then stacked with intervening baffles b, baffles c being rotated 180 degrees relative to baffles a so as to build up an interdigitated delay line the crosspieces 4 forming the interdigitating members of the delay line, as shown in Fig. 7. Each crosspiece has a part annular protrusion 6 with a beam passage aperture. By designing the baffles so d # R/2 (=diameter of aperature in baffle 2 Fig. 1 and R=internal radius of baffle 1) the dispersion curve can be 'shifted' bodily into the desired frequency range. By giving the spacing baffles diametrically opposite inward projections Fig. 1A (not shown) the dispersion curve can be changed in shape in such a way that the dispersion of the desired forward wave is very small (therefore large bandwidth as is required). If the projections overlap the crosspieces neither the backward wave fundamental nor the first forward spatial harmonic travelling wave components can interact with the electron beam so the forward wave first spatial harmonic is used. The shape and dimensions of the baffle plates determines the shape of the dispersion curve, whereas the shape and dimensions of rings 2 determine the position of the dispersion curve but do not substantially affect its shape. While design is such that the fundamental is a backward wave the first forward spatial harmonic travelling wave component is used for interaction. The crosspieces may take up one half the area of the waveguide, Fig 2 (not shown) and the beam may be flat, Fig. 3 (not shown) instead of cylindrical Figs. 1, 2, the crosspieces of the baffles having a flattened central portion to allow the beam to pass over it. For a cylindrical beam the baffle crosspiece centres may have a semicircular depression. If the edges of the crosspieces are suitably positioned the spacing baffles can be omitted; this also applies in Fig. 5 (not shown) where a flat beam can be used, a rectangular recess being formed in the centre of the edges of the crosspieces. In Fig. 6 (not shown) which uses a cylindrical beam, the baffles have crosspieces having a sectoral angle of 200 degrees to reduce the dispersion of the first forward travelling wave spatial harmonie component (and therefore a wide band width). Chestrong magnetic coupling through the baffle slots creates a fundamental backward wave. The dispersion curve can also be changed in shape by suitable choice of the baffle sectoral angle. Waveguide imput and output feeders can be in the wall of the delay line by providing additional holes in line with one another in the outer parts of the baffle plates. The baffle crosspieces can be used for electrostatic focusing. Periodic magnetic focusing by an axial magnetic field which alternates both in time and space can be used. In another arrangement the crosspieces and adjacent annular parts of the baffles completing the apertures in the baffles are made of magnetically soft material and the annular spacers and other parts of nonmagnetic material for the production of the space periodic magnetic field, which in this case is still axial but is produced by two diametrically opposite magnets radially magnetised in opposite senses the baffle plates a and c being connected alternately to the south and north poles of the magnets. In another embodiment Fig. 8 there are a plurality of delay lines (forming an amplifier and/or limiter) surrounding a central delay line forming a backward wave oscillator preferably for mm waves which feeds the outer delay lines through a symmetrical conical distributer at the gun end of the tube having V shaped notches in its outer surface to match the outer delay lines and having its apex secured to the central delay line. The crosspieces may be as in any of Figs. 1-6. A hollow annular electron beam passes outside the outer delay lines.