Patent GB-714832-A (GB)
Title: Improvements in or relating to microwave amplifiers
IPC: H01J25/38
Filed: 1950-06-13
Abstract: 714,832. Travelling wave tubes. WESTERN ELECTRIC CO., Ltd. June 13, 1950 [June 17, 1949], No. 14713/50. Class 39(1) [Also in Group XL(b)] In a travelling wave amplifier designed for spatial harmonic operation, and having a transversely slotted wave-propagating structure, the ratio of the slot width to the distance between the centres of adjacent slots is not greater than threetenths. The value is exactly three-tenths for the first spatial harmonic mode, and has corresponding smaller values for the higher harmonics. and this design ensures maximum interaction between the beam and the wave. In one embodiment, Fig. 1, the copper block 10 has transverse quarter-wave slots 11 to slow up the wave and three longitudinal slots 12 of about half this depth to increase the area of interaction between the wave and beam. The interior cross section of the block is rectangular and the fins 13 are also rectangular. Inout and output are led in and taken off via coupling slots 14, 21, quarter wave impedance transformers 25, 31 and waveguide sections 26, 24; 32, 36 separated by molybdenum cups 29,35 brazed or soldered to block 10 and having glass windows 28, 34 sealed thereto. The guides and impedance transformers are all of rectangular section. Annular slots 27, 30, 33, 37 serve as radio frequency chokes and the ends of guides 24, 36 are enlarged where they are secured to block 10 so as not to touch molybdenum cups 29, 35. The cathode 16 comprises a heater coil 20 embedded at one end in the end face of a rectangular sectioned cathode body coated with oxide and the cathode and heater leads extend normally to the plane of the paper in Fig 1. The active area of the cathode may or may not be shaped to conform with the cross section of the beam. Control grid 17 and accelerator grid 18 are flat molybdenum plates with tungsten wire grids extending across the central apertures therein. The wave transmission path is shorted at both ends along sections 15, 22 by making the end fins extend right across the interior opening in block 10. The three longitudinal slots are however left and these together with two larger side slots determine the form of the beam which is thus divided into five parts of elongated cross section. Electromagnet poles 55. 56 supply a longitudinal focusing field, and a collector 23 is provided. The leads to cathode 16, control grid 17 and collector 23 are tungsten rods such as 44, 49 and are taken out through three seals, two of which are shown in Fig. 1, comprising copper sleeves 45, 50, molybdenum sleeves 46, 51 brazed to the copper sleeves, and glass caps 47, 52. No iron alloy is used in any of the seals of the device so as not to affect the magnetic focusing field deleteriously. The wave and beam velocities are such that the beam travels between the midpoints of two adjacent slots while the wave travels this distance plus an integral number of wavelengths, preferably one wavelength. The axial component of the wave vanishes in the metal walls between the slots, so that interaction is substantially confined to the slots and each electron meets a similarly phased electric vector each time it crosses a slot. The wavelength and slot spacing may or may not be integrally related. The device normally operates at about 1.25 cm. but mav be constructed to operate at 6 mm or less. The wave transmission path is so designed that the phase velocity is independent of small frequency variations, and to achieve this it is shown that the phase velocity of the notional wave should be equal to the group velocity of the 'real ' travelling wave. The input coupling to the device may be modified as shown in Fig. 1E in' which the end slot 14 is half the width of the other slots 11 and is coupled to guide 26 by rectangular hole 57 and two quarter wave matching sections 58, 59. The centre of the matching system is slightly displaced to the left relatively to slot 14, and the system extends through the lower part of the device instead of the upper part as in Fig. 1. The output circuit may be similarly constructed. In another embodiment, Fig. 2 (not shown) the interior cross section of the block 10 is H-shaped and half wave flat copper rods are arranged at axial intervals across the cross-arm of the H to form the wave transmission path. The beam passes on both sides of the rods in two parts of elongated cross section. In another embodiment, Fig. 3, the interior cross-section of the block is circular and a transversely and longitudinally slotted copper rod 71 is arranged along the axis. The end sections to which rod 71 is brazed are as shown in Fig. 3C; they have slots 72 which divide the annular beam into four parts. The fins 13 stop off further parts of the beam so that the final cross section is as shown by the dotted lines 19, Fig. 3B. The accelerating grid 73 covers the slots 72. The input and output couplings are via tapered waveguide sections 74, 75. In another embodiment. Fig. 4 (not shown), the transmission path structure is the inverse of that of Fig. 3, the fin structure extending from the inner wall of block 10 and the central conductors corresponding to 71, Fig. 3, being a smooth cylinder. The fins are annular and are brazed to the inside of block 10. The annular beam is sub-divided in a manner similar to that of Fig. 3. Specifications 652,155 and 669,473 are referred to.