Travelling-wave tubes

`Travelling-wave tubes' is the name of a recently developed class of electron tubes, operating in the microwave range of frequencies, which amplify by virtue of continuous interaction between an electron stream and a slow-travelling electromagnetic wave. The wave may be propagated along a retarding structure such as a helix or a corrugated waveguide, or it may be a space-charge wave in a second electron stream. The history of the discovery and the development of the travelling-wave tube is traced and a simple theory of operation outlined. A more complete theory according to J. R. Pierce is sketched by which can be calculated the effects of attenuation, of non-synchronous velocities and of space-charge effects. Recent developments relevant to the helix-type travelling-wave tube are described, such as `Brillouin'-focusing of electron streams in axial magnetic fields, or noise properties of electron streams. Other forms of travelling-wave tubes described include Millman's space-harmonic type of tube operating as amplifier at a wavelength of 6 millimetres, the electron-wave tube, and the travelling-wave magnetron amplifier which combines the high efficiency of the magnetron with its amplifying function. Wherever possible attempts have been made to give explanations of the modes of operation of the various devices in simple physical terms.