Patent GB-772001-A (GB)
Title: Improvements in or relating to electron discharge devices of the travelling wave type
IPC: H01J23/26
Filed: 1954-03-23
Abstract: 772,001. Travelling wave tubes. WESTERN ELECTRIC CO., Inc. March 23, 1954 [March 30, 1953; June 26, 1953; June 26, 1953], No. 8458/54. Class 39(1). A travelling wave tube has two electrodes between which the beam passes, one electrode being equipotential and the second electrode being divided into a plurality of parts having potentials varying with distance about the potential of the first electrode so that a spatially alternating electrostatic field is set up for focusing the beam, one of these electrodes propagating a slow wave parallel to the beam. The electrostatic focusing is achieved by a structure comprising two parallel or concentric electrodes one of which is in two parts or two sets of parts at different D.C. potentials, V, and V2 say, and the other of which is continuous and at a D.C. potential V3 lying between the potentials V1 and V2. In the case of an annular beam the potential V3 of the continuous electrode is made less than (V1 + V2) / 2 to give a net outward force at the inner edge of the beam and so to stabilise the inner boundary of the beam. For good focusing the current density across the beam should increase from the axis outwards and this is achieved by using a hollow cathode, Fig. 9, or by converging the beam from a conventional cathode in the way described with reference to Fig. 4, Fig. 5 (not shown). A lengthy mathematical treatment is given. In one form, Fig. 1, which is a backward or forward wave amplifier operating in a spatial harmonic mode, the wave propagating bifilar helix 16, 17 also serves in conjunction with electrode 19, as an electrostatic focusing structure. The beam is annular and the bifilar helix is wound on a ceramic support 18 ; alternatively the bifilar helix and electrode 19 may change places, in which case the helix is carried by dielectric rods. The feeder lines 14, 15 ; 23, 24 and the bifilar helix are balanced lines, i.e. the h/f potentials are equal and opposite at corresponding points of the two conductors. Electrode 19 can be a resistive coating on the tube wall. For forward wave operation loss material may be provided to minimise self oscillation. The circumference of one turn of the helix is less than # so that the h/f field reverses at successive gaps and this is stated to favour spatial harmonic operation. A magnetic focusing field is not usually required. A backward wave oscillator may be similarly constructed except that the collector end of the helix has a reflectionless termination comprising lossy material sprayed on to the ceramic support, Fig. 2 (not shown). The loss may increase gradually over several turns to provide a broad band termination. A signal source may be connected to vary the accelerating potential to frequency modulate the output. The wire of the helices can be of circular or ribbon section. The gun may comprise a series of rings 82, Fig. 5, of graduated diameters whose potentials (a) alternately increase and decrease, and (b) the mean value of which gradually increases, to give gradual acceleration as well as convergence. The inner surfaces of the rings are sloped to be parallel to the beam envelope. The beam is annular, a set of disc electrodes concentric with rings 82 being added. The disc electrodes may be of progressively decreasing diameter, Fig. 5 (not shown) or of constant diameter, Fig. 4, and are slightly negative to the rings 82 to stabilise the inner boundary of the beam. The beam forming electrode may also have a central disc, Fig. 5 (not shown). In Fig. 4 an annular beam passes between a slow wave helix 86 and a focussing bifilar helix 84, 85. The ends of the helix 86 are of increasing pitch for matching. In another form, Fig. 9, focusing is achieved by two sets of rings 127A, 127B at different D.C. potentials surrounding a wave propagating helix 126 which may be wound of wire or tape. The cathode comprises a toroidal cavity 121B coated with emissive layer 121C, heater 122 and heater sheild 123 for producing an annular beam. The control electrode 124 and accelerating electrode 125 are annular mesh grids. The helix 126 is connected to the inner conductors 128A of coaxial feeder lines 128, 129 via matching portions of increasing pitch and decreasing diameter. The invention can also be applied to linear magnetron type travelling wave tubes, Fig. 12. A flattened wave propagating helix 141 faces two sets of bars 142 at different D.C. potentials. The gun 148 is located in a field free region and produces a ribbon beam the magnetic field is produced by permanent magnet poles 143, 144. In the transition region between the gun and pole-pieces the potential of the bars is progressively increased away from the gun to match the increasing fringing magnetic field so that the ratio is maintained constant. Alternatively, the spacing between the helix and bars may vary in the transition region. The transverse electric field is adjusted to a value so that there is no net transverse force acting on the electrons. In general, other forms of delay line can be used. The bifilar helix can be replaced by a multifilar helix. Specifications 701.034, 734,479, 767,446, 772,000, 772,003 are referred to.