Microwave Electron Tubes – High power tubes

The grid tubes presented in [E 1 620] suffer from the same limitation as solid-state devices: the time taken for an electron to travel the interaction structures (the cathode distance – grid in this case) must be small compared with 1/f, where f is the signal frequency. However, unlike a transistor where these structures can reach sub-micron sizes, in a grid tube it's impossible to go below distances of the order of a hundred microns.

Klystrons suffer from a similar problem (the first klystrons had grids at the entrance/exit of the sliding tunnels), but their very different operating principle (speed modulation rather than density modulation) results in superior frequency performance.

However, most microwave tubes exploit a completely different idea, based on the synchronism between a beam of electrons and an electromagnetic wave following the same path. The idea is as follows: if an electron is placed in an RF electromagnetic field in such a way that it undergoes the influence of a decelerating field (E>0) for a "relatively long" period of time, meaning more than 1/f, then it will radiate its energy. But how can an electron accelerated in a vacuum reach a speed close to the speed of light in a vacuum? This is the subject of this article and the [E 1 622] article on TWTs. If the electron is moving perfectly rectilinearly at speed v_e, this synchronism condition implies that v_e is equal to v_ϕ the phase velocity of the wave, knowing that v_ϕ is equal to ω/β where β the wave vector and ω the pulsation. If the rectilinear motion of the electron is superimposed by a transverse oscillating motion (at pulsation Ω), conservation of momentum requires replacing v_e by v_e + Ω/β in the synchronism condition. The first condition applies to magnetrons, TWTs or EIKs; the second to gyrotrons, but also to free electron lasers (FELs).

The synchronism condition has a slight subtlety: for the electron to yield maximum energy to the wave, it must be slightly violated (the electron must be barely faster than the wave): this is a very general principle in physics, known in optics as the Cherenkov effect.

In all microwave tubes, a beam of electrons is emitted...