Patent GB-825546-A (GB)
Title: Improvements relating to travelling wave tubes
IPC: H01J23/24
Filed: 1956-01-16
Abstract: 825,546. Travelling wave-tubes. GENERAL ELECTRIC CO. Jan. 25, 1956 [Jan. 25, 1955], No. 2497/56. Class 39(1). A travelling wave tube comprises a coaxial line modified to have delay and filter properties, by loading it with two coaxial arrays of spaced opposed tubular conductors 24, 25 &c. and 22, 23 &c., and a tubular electron beam is projected between the arrays parallel to and around the inner conductor of the coaxial line to interact with the retarded wave travelling therealong ; input and output means 39, 42 are coupled respectively to the end outer conductors. In one form, Fig. 1, an annular electron beam from indirectly heated cathode 10 and accelerated by electrode 17 passes between two sets of conductive cylinders 22, 23 ; 24, 25 the inner set of which are coupled to the inner conductor 28 and the outer set to the outer conductor and radiation shield 29 by sets of light radially arranged conductive rods 26, 27, the rods preferably extending across the gaps between the two sets of cylinders so as to act also as straps. The strapping eliminates the undesired transverse electric field mode (low pass mode) while permitting the axial electric field mode (high pass mode). The field component is confined to the region of the radial rods so that the input and output couplers 39, 42 which have a circumferential position midway between two rods are decoupled from intermediate sections of the delay line. The cylinders 24, 25 are sealed to glass envelope portions 35 and to glass cups 34, 37 housing the gun and collector 18 respectively. Shunt attenuation is provided by one ore more e.g. two tapered cards 44 of glass or ceramic disposed in an inductive region of the line and coated with lossy material. The width and/or distribution of the loss material are reheated to get desired loss characteristics. An axial magnetic focusing field is provided. Alternatively outer conductor 29 can be part of the vacuum wall. The beam channel is as narrow as possible so as to get a constant axial electric wave field over the beam cross-section. There may be only one or two radial rods 26, 27 for filter section but preferably there are more than two. In a modification the radial rods can be replaced by two sets of discs but the resulting structure has a narrower pass band ; holes can be drilled in the discs to obtain optimum shunt inductance ,Fig. 4 (not shown). It is shown that the delay line sections are # filter sections and that by special design optimum beam-wave coupling can be obtained. A mathematical theory and a numerical example are given. In a further form, Fig. 5, there are two radial rods 77, 78, a third rod in a region of no field being extruded to form the inner conductor of a coaxial line feeder. The rods 77, 78 can be of conductive or high resistance material or can carry members 79, 80 of high resistance material. The rods may be of glass or ceramic material sprayed with loss material instead of being wholly of high resistance material, the loss coating being remote from the beam to give shunt loading. The coaxial live feeder may be replaced by a waveguide and the feeder may be coupled to the end filter section or to an intermediate filter section. The feeder coupling by means of a radial rod or a loop, Fig. 6 (not shown), reduces the band width of the arrangement since it gives substantial coupling to intermediate as well as to end sections. Some loss coatings may be shorter than others. The delay line can be square or polygonal in crosssection. The tubes are used as relays in communication systems. Specification 825,545 is referred to.