Patent GB-816411-A (GB)
Title: Improvements in and relating to travelling wave tubes
IPC: H01J23/28
Filed: 1957-04-25
Abstract: 816,411. Backward wave oscillator tubes. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. April 25, 1957 [April 25, 1956], No. 13232/57. Class 39(1). In a backward wave oscillator operating in a spatial harmonic mode having a sole electrode opposite a delay line, the length of the delay line elements perpendicular to the surface of the sole electrode gradually decreases from the cathode end towards an intermediate element of the delay line all the delay line elements being spaced from the sole electrode by the same amount. In this way the "coupling resistance between beam and delay line is made less at the end of the delay line adjacent the cathode than at an intermediate region thereof. The object is to reduce the heating of the delay line by electron bombardment. This is achieved by making the coupling resistance lower at the end adjacent the cathode as aforesaid, since this requires the coupling between beam and wave to be greater at this portion so that the delay line fingers are made larger and can dissipate more heat caused by electrons bombardment and can carry more high frequency power to the output. As shown in Fig. 1 the length of the the fingers gradually decreases from the cathode end to the centre and then increases again for the second half of the delay line. The beam is most likely to bombard the initial part of the delay line to the fingers in this part are essentially made longer than the others. The distance of the sole plate from the delay line fingers is made constant over the whole length of the delay line, and the delay line, preferably interdigilated as shown in Fig. 1, is of constant pitch. In Fig. 1 the interdigitated delay line is made up of two combs fixed to the inner surface of the envelope which forms the delay line base. The sole plate 21 is supported by three ceramic rods 22, 23, 24 soldered or brazed to one tube end cap. The delay unit envelope spacing may be constant or the envelope can be concentric with the inner delay line surface but eccentric to the outer delay line surface so that the spacing delay line-envelope is greater at the centre of the delay line than at its ends ; in the latter case the delay line is obtained by milling a cylinder whose outer and inner surfaces have parallel but non-coincident axes. 12, 13 and a permanent or electromagnet forms the beam, 12 being the webvelt electrode. The part of the delay line between 16 and 17 has a lossy coating or other internal or external loss means. In another embodiment Fig. 3 the delay line is a comb or ladder supported at one or both ends respectively by lateral walls secured to base 81. The diameter of the delay line rods 82 ... 87 decreases progressively from the cathode towards the centre portion of the line and then increases again to the collector and the dimensions and spacing are such that the ratio of the capacities from each rod to the next are constant along the delay line, the spacing from the rods to the sole plate being constant. The tube of Fig. 3 is linear but may be circular and conversely the tube of Fig. 1 may be linear instead of circular. Specification 699,893 is referred to.