Patent GB-677986-A (GB)
Title: Improvements in electronic discharge tubes
IPC: H01J25/38
Filed: 1948-05-03
Abstract: 677,986. Travelling-wave tubes. PHILCO CORPORATION. May 3,1948 [July 18, 1947], No. 12123/48. Class 39 (i). [Also in Group XL (b)] In a travelling-wave electron discharge device the velocity modulating and energy extraction circuits are constituted by separate helically-coiled hollow pipe wave-guides 30, 34, Fig. 1, and the output guide is of progressively diminishing pitch in the direction of travel of the beam to maintain synchronism between beam and wave. In Fig. 1 the wave-guides are bounded by metal strips 31, 36 wound inside a metal cylinder 33; the guide 30 is of uniform pitch. The collector potential may correspond to the reduction in beam velocity when it has given up energy to the wave. The coupling to input and output circuits is via coaxial lines 39, 44, and a solenoid 51 wound on insulator 52 focusses the beam; a magnetic shield (not shown) may also be provided. The guides 30, 34 may be provided with matched terminations at both ends consisting of uniformly-tapered helical blocks of lossy material 54, such as ceramic containing powdered graphite, having a circumferential length of at least one wavelength at the lowest frequency of operation. Alternatively the end turns of the guides may be tapered and covered with the lossy material The guides 30, 34 are excited in the TE 0  mode with an axial electric field component which is maximum at the inner edges of the strips 31, 36 and diminishes sinusoidally to zero at the inner surface of the cylinder 33, and the wave travels around one complete turn, in one half-cycle. The space between the input and output guides serves as a drift space. The diameter of glass envelope 21 is such that considered as a separate wave-guide its cut-off frequency is considerably above the operating frequency band. The device may function as an oscillator by providing an external feed-back path between coaxial lines 39, 44 which contains a tunable resonant cavity to control and stabilize the frequency, or alternatively by producing suitable wave reflections inside the device. It may also be used as a frequency converter, modulator or detector. The characteristic impedance and other electrical constants of the guide 34 of variable pitch may be rendered constant along its length by inserting a tapered helical block of dielectric material therein or by varying the outer diameter of the helix along its length. In a modification, Fig. 2 (not shown), the input and output guides have their turns spaced apart. The wave-guides have lossy matched terminations similar to those shown in Fig. 1, and further lossy material may be provided between the two guides and between the turns of the wave-guides. The spaces between the turns of the guides act as drift spaces. In a further modification, Fig. 3 (not shown), the helical guides are enclosed within the evacuated envelope, and insulating seals are provided in the input and output coaxial lines to maintain the vacuum. In further modifications, the guides may be wound with a large pitch if they are filled with a solid low loss dielectric. They may be formed of a resistive material instead of metal and a D.C. potential gradient set up along their length so as to improve the efficiency.