In cross-field tubes — also called ‘M-type’ tubes after magnetrons — the electron beam moves under the influence of perpendicular static electric and magnetic fields. As a result, its drift motion is perpendicular to both fields, while its average drift velocity depends only on the ratio of the absolute value of these fields. Such a beam is made to interact with a space harmonic of the electromagnetic wave propagating along a slow-wave structure. Since the drift velocity of the beam is constant, the beam does not give up its kinetic energy to the wave, as in the case of linear beam tubes — ‘O-type’ tubes (as opposed to M-type) such as klystrons, TWTs, etc. — but, instead, its potential energy, so that it remains synchronous with the wave while interacting with it. For this reason, high efficiencies are obtained.
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