One uses a slow-wave circuit called a helix for propagating the RF wave for electron-RF field interaction, and the other employs a series of staggered cavities coupled to each other for wave propagation.
A technique known as radio-frequency (RF) current drive employs electromagnetic radiation to generate a steady-state current.
This discussion focuses on those electron tubes that serve as circuit elements, functioning as rectifiers, microwave RF sources, and amplifiers.
The amplitude of the RF voltage between the grids is determined by the amplitude of the input signal.
This causes the electrons to bunch and results in the density modulation of the beam, with the electron bunches representing an RF current in the beam.
The advantage of fast-wave devices is that the RF circuits are large compared with the wavelength of a signal.
The high-voltage AC currents used are the main reason that fast-wave devices produce exceedingly high RF power levels, up to millions of watts at very high frequencies (more than 100 GHz).
When the bunches interact with the traveling wave, the electron energy is converted to RF energy and results in amplification.
The main elements of a traveling-wave tube (TWT) are (1) an electron gun, (2) a focusing structure that keeps the electrons in a linear path, (3) an RF circuit that causes RF fields to interact with the electron beam, and (4) a collector with which to collect the electrons.
In order to achieve this interaction, the diameter and pitch of the helix must be such that the RF wave traveling on the helix wire at the speed of light (about 300,000 km, or 186,000 miles, per second) is slowed down in its axial travel to be in synchrony with the velocity of the electrons in the beam.
This circular motion is exploited in many electron devices for generating or amplifying radio-frequency (RF) power.
