GB754383A - Improvements in or relating to travelling wave tubes - Google Patents

Abstract

754,383. Travelling-wave tubes. WESTERN ELECTRIC CO., Inc. March 26, 1954 [April 3, 1953], No. 8910/54. Class 39 (1). The wave propagating circuit of a travelling- wave tube comprises a coaxial line 44, 45, Fig. 5, the inner conductor, along the interior of which the beam flows, being provided with a plurality of pairs of diametrically opposite slots 46, alternate pairs of slots being in the same circumferential positions and adjacent pairs of slots being displaced by 90 degrees so that the arrangement functions as an electrostatic focusing device for focusing the beam successively in different axial planes. It is shown that the system is equivalent to a " quadrupole " system of electrodes, Fig. 1 (not shown), and focuses the beam alternately in two mutually perpendicular planes, so that magnetic focusing means is not necessary. The number of slots around the circumference may be increased to obtain " sextipole " or " octopole " focusing. The circuit is particularly suitable for spatial harmonic operation in a mode having a positive or negative phase velocity, i.e. as a " forward " or " backward " wave amplifier, Fig. 5, or as a " backward " wave oscillator, Fig. 6 (not shown). The coaxial line conductors may be corrugated to slow up the wave. The analogy with prior art " quadrupole " magnetic focusing arrangements is discussed. In the form shown the slots 47 are rectangular and of equal size; their length is slightly less than the axial spacing between adjacent pairs of slots which spacing is small compared to the operating wavelength. The inner conductor 44 is positive with respect to outer conductor 45 and the slots 47 in the inner conductor each extend over about 90 degrees of the circumference. The mathematical conditions for synchronism are given for " forward " and " backward " waves of the first spatial harmonic modes. For " forward " wave amplification a coating of loss material, e.g. colloidal graphite, is provided on either of the coaxial line conductors. In a backward wave oscillator, Fig. 6 (not shown), the downstream end of the line has a reflectionless termination inside the tube comprising an annular wedge of dielectric material coated with lossy resistive material. In such a tube the frequency depends on the beam velocity so that if a signal source is connected between the cathode and the inner conductor of the line a frequency modulated output is obtained. The shapes of the line conductors and slots may be modified if desired.