Patent GB-825545-A (GB)
Title: Improvements relating to travelling wave tubes
IPC: H01J23/48
Filed: 1956-01-25
Abstract: 825,545. Travelling wave tubes. GENERAL ELECTRIC CO. Jan. 25, 1956 [Jan. 25, 1955], No. 2496/56. Class 39(1). The beam from axial electron gun 11, Fig. 1, to collector 27 passes through an envelope portion consisting of alternate conductive cylinders 17 . . . 21 and insulation 23. . . 26, the former forming, together with outer cylinder 30 to which they are connected by conductive and/or attenuating material 31. . . 35, a delay line having input and output means 42, 44 connected respectively to end cylinders 17, 21. In Figs. 1 and 2, members 17 . . . 21 have a diameter and length designed for optimum beam coupling and are sealed to glass or ceramic cylinders 23 . . . 26, glass or ceramic gun housing 22 and water cooled collector 27. The input and output feeders comprise coaxial line sections 43 coupled to the delay line by a coupling line 41, 42. Attenuator cards 46 . . . 48 extend through and contact end plates 41, 45 and consist of a tin chloride coating on a glass plate or a titanium dioxide coating on a ceramic plate, and their thickness can be varied in the axial and radial direction to give optimum stability. The conductor 30 is of brass or copper and the conductive legs 31 of successive-members 17-21 all lie in the same plane ; the length of the shunt attenuators is < n#/4 where n is any odd number, and the legs have a length less than #/4. The feeders are coupled to an end or to an intermediate filter section, the coupling not substantially affecting the field in the other filter sections and the device is designed to give high constant gain over a wide frequency band. An axial magnetic focusing field is provided. In further embodiments the legs 31 are made of high loss material or of dielectric material coated with colloidal graphite ; coupling to the feeders may be via a coaxial line section or a probe may be inverted in a waveguide, Figs. 5, 6 (not shown). To ensure stabilization against oscillations the resistive or lossy component is introduced in an inductive or capacitive part of the line, preferably the former, Figs. 2, 3, 5 (only Fig. 2 shown) using loss members in inductive parts of the line. In other forms a resistance element or high loss rods 70 may be provided in the gaps between members 17-21 (in the capacitive regions) but it is stated that since the former become saturated with induced beam current they cannot attenuate properly, Fig. 7 (not shown). Preferably the loss is introduced in the inductive parts of the line in a plane near or between the legs 31 or a lossy dielectric or dielectric coated with lossy material such as colloidal graphite, alternatively the legs 31 are of stainless steel or a dielectric coated with a lossy film, Fig. 8 (not shown). The arrangement of Fig. 8 also has a flatter loss-frequency charactereristic and the propagation characteristic is less affected by the presence of the loss element than in Fig. 7. The reduction in efficiency due to low frequency high gain fields saturating the attenuator is said to be overcome by reducing the area of lossy coating on one or more cards. The loss can be only in the vicinity of the legs 31 or can extend over the entire length of the cards.