GB734479A - Improvements in or relating to travelling wave tubes - Google Patents

Abstract

734,479. Travelling-wave tubes. WESTERN ELECTRIC CO., Inc. Dec. 16, 1953 [Dec. 23, 1952], No. 35017/53. Class 39 (1). A travelling-wave tube has a delay line comprising a hollow wave-guide from opposite walls of which U-shaped wire elements 31, Fig. 1, extend in a linear array so as to overlap in an interdigitated pattern, and means for projecting a beam of electrons through the overlapping portions of adjacent wire elements. The device may be a backward wave oscillator, as shown, or an amplifier using forward or backward wave modes, Fig. 3 (not shown). For a backward wave device the spatial harmonic selected for interaction with the beam is a forward component of the wave which has a backward energy flow, and in the backward wave oscillator the wave is initially set up by noise currents in the beam. The forward wave amplifier also uses a spatial harmonic mode. The rectangular sectioned guide 18 is terminated non-reflectively at the collector end by a wedge 22 of dielectric coated with lossy material, e.g. powdered graphite, and at the gun end is connected to output guide 36 via a tapered im, pedance-matching section, right-angle bend 25, guide 24 and guide 26; guides 26, 36 have capacitative coupling flanges, and all the guides are of rectangular section. The cylindrical beam is focused by solenoid 35. Brillouin-type flow may be used, as described in Specification 697,550. The impedance matching at the output end is assisted by tapering the lengths of U-shaped elements 31. The elements 31 have a length, i.e. the height of the U, less than n #G/4 where n is any odd number, i.e. avoiding multiples of #G/4 where the elements are resonant. The electron transit time between adjacent elements is about a half period of the wave. In making the delay line, gold-plated molybdenum wire is wound as a helix on a steel mandrel of rectangular cross -section and the wound mandrel placed in a groove in a copper block, after which the wire is brazed to the edges of the groove. The brazing operation is facilitated by gold plating the edges of the groove. A taper may be ground at either or both ends of the wound helix for impedance matching. The steel mandrel is then dissolved chemically. Two such block and element assemblies are brazed to two copper'side plates to form the complete delay line. The tubes may operate at millimetre wavelengths.