Patent GB-895299-A (GB)
Title: Duplex travelling-wave tube amplifier
IPC: H01J25/34
Filed: 1958-08-11
Abstract: 895,299. Travelling-wave tubes. SYLVANIA ELECTRIC PRODUCTS Inc. Aug. 11, 1958 [Aug. 19, 1957], No. 25797/58. Class 39 (1). A travelling-wave tube comprises at least two separate, parallel, laterally spaced slowwave structures coupled respectively to input and output connections and spaced apart in a direction transverse to the direction of propagation of waves from one end to the other of each structure, so as to be traversed in sequence by an electron stream having a velocity component in the same direction as and approximately equal to the phase velocity of the waves on at least part of each slowwave structure. In one embodiment, two flattened helices 9, 10, Fig. I are wound on insulators 11, 12 and are coupled to the input and output of the tube. The electron stream is provided on each side of the helices by an indirectly heated cathode comprising two strips 27, 28, Fig. 2 and focusing electrode 29. A magnet 46 provides a magnetic field at right angles to the electron stream which is thus bent at an acute angle to the axial direction of the helices, the velocity component of the stream in the axial direction being slightly greater than the phase velocity in the helices for the required mode of operation. A ferromagnetic strip 23 concentrates the field over the deflection space, and a metallic strip 22 shields the helices from each other. Beams 38-45, Fig. 1 are shown as being typical of the whole stream. In operation an input wave travelling along input helix 9 from left to right interacts with beam 38 which becomes velocity modulated as it travels towards output helix 10, an r-f signal being induced in this helix by the modulated beam. In the meantime, the input wave travels along helix 9 to the point where interaction with beam 39 begins, and this beam which also becomes velocity modulated in traversing the interhelix space, arrives at helix 10 at the correct moment to interact in additive phase relationship with the r-f field induced by beam 38 and travelling from left to right along helix 10. Thus the separate induced wave components reinforce each other and produce correspondingly stronger r-f fields to interact with the subsequent beams, the power efficiency associated with which becomes progressively greater. The process continues until saturation effects set in. To counteract saturation, control electrode 29 may be twisted so that the electron stream although still parallel to the helices at the input end of the tube (Fig. 2) gradually slopes away from the helices towards the output end, Fig. 3 (not shown). The envelope of the tube may serve to collect the electron stream and be provided with cooling fins 4-8. Insulators 11, 12 have lossy coatings 16, 20 at the output and input ends respectively to prohibit backward and reflected modes. Tuning may be effected by altering either the accelerating potential of the stream, or the magnetic field, or both. Another embodiment employs a number of suitably aligned electron guns providing a series of beams instead of a sheet stream, the magnetic field being unnecessary. A further embodiment uses a sheet electron stream at right angles to the axial direction, the helices in this case being skewed. Various methods of operating the tube, other than as a broad band amplifier are given, for example as a backward wave oscillator, when helix 10 may or may not be used, or as a narrow band amplifier with feed back.