Patent GB-740998-A (GB)
Title: Travelling wave tube
IPC: H01J25/40
Filed: 1953-04-28
Abstract: 740,998. Travelling-wave tubes. NATIONAL RESEARCH DEVELOPMENT CORPORATION. April 28, 1953 [May 17, 1952], No. 11762/53. Class 39 (1). The delay line of a backward wave oscillator comprises a transversely and longitudinally slotted block 10, Fig. 1, the transverse slots being resonators and electron beams passing through the longitudinal slots, a ribbon helix, Fig. 2 (not shown), or an interdigital line, Fig. 3 (not shown), comprising two rows of interdigital fingers whose length tapers at the output end. The mode used is a spatial harmonic mode in which the wave travels an integral number of wavelengths less the distance between mid-points of adjacent slots 11, Fig. 1, while the beam traverses the latter distance only, and the wave is initially excited by noise components in the beam. Since the structure is dispersive, tuning can be effected by varying the beam velocity. The slow wave structure may be a wave-guide 10, Fig. 1, one wall of which is formed with transversely and longitudinally slotted fins 13. Terminating slot 14 of smaller depth and width and tapered wave-guide 25 provide impedance matching to output guide 24, and the vacuum is maintained by glass window 28 sealed to molybdenum cup 29. Slots 27 and 30 are chokes and guides 24, 25 are of rectangular section. Section 15 comprises a number of parallel bars which split up the beam into five parts and short-circuit the end of the structure for radio frequencies; the structure at the right-hand end 22 is similar. Cathode 16 contains heater 20, and control grid 17, accelerator 18 and collector 23 are flat rectangular molybdenum plates with grids across central rectangular apertures therein. The slow wave structure is non-reflectively terminated at the collector end by two tapering wedges of loss material comprising ceramic material coated with colloidal graphite, Fig. 15 (not shown), or by a separate non-reflectively terminated wave-guide (not shown). An electromagnet with poles 55, 56 supplies a longitudinal focusing field. The tungsten cathode leads are brought out perpendicular to the plane of the paper through a copper-molybdenumglass seal, Fig. 1D (not shown) and similar seals are used for the control grid 17 and collector 23, as shown. Frequency modulation is effected by modulating the accelerating voltage as indicated by source 57. In another embodiment, Fig. 2 (not shown), an annular beam flows inside and/or outside a ribbon helix, Fig. 2 (not shown) or similarly-shaped ribbon. The helix may be formed by cutting a helical groove in a hollow cylinder, and the pitch of the groove may be tapered at the gun end to give a better impedance match to the output guide. The non- reflecting termination at the collector end is a ceramic block having a cylindrical outer and a conical inner surface, the inner surface being coated with colloidal graphite. In another embodiment, Fig. 3 (not shown) an interdigitated delay line is used. The magnetically focused annular beam passes between two rows of interdigitated fingers extending from the broader walls of a rectangular wave-guide. The fins decrease in length near the output guide for impedance matching and interleave over the main part of the line. A 45 degrees deflector plate diverts the waves into the output guide which extends at an angle of 90 degrees with the main part of the tube. The collector end is non-reflectively terminated by two inclined ceramic plates coated with loss material. Since the field between fingers reverses at each successive gap the wave must traverse an integral number of half-wavelengths less the average distance between successive gaps while the beam traverses only the latter distance.