A microwave linear-beam vacuum tube that consists of an electron beam and a slow-wave structure. The electron beam from an electron gun is focused along the length of the tube by a constant magnetic field. The slow-wave structure is often a helical coil wound round the tube (see diagram). An applied radiofrequency (RF) signal is propagated along the coil and produces an electric field directed along the central axis of the coil. This axial field progresses with a phase velocity much lower than the speed of light (in the ratio of coil pitch to its circumference – the RF signal must travel around each loop of the coil to progress along the length of the tube), hence the description ‘slow-wave’. The electrons in the beam interact with the axial field, and transfer energy to the RF signal. The RF signal is thus amplified. This is known as forward-wave amplification.

The backward-wave oscillator is a form of linear-beam travelling-wave tube in which the optimum transfer of power produces RF waves in the backward direction, i.e. the group velocity and phase velocity differ by 180°. A beam current of sufficient magnitude interacts with the slow-wave structure to produce RF oscillations that are delivered as microwave power at the electron-gun end of the structure. The minimum beam current required to produce oscillations is the start-oscillation current. At currents below this value the tube may be used as an amplifier by supplying an RF-wave input at the collector end of the structure. The interaction efficiency of this type of tube is increased by using an electron beam with a hollow cross section. This is achieved by magnetically confining the electron flow from the cathode.

Travelling-wave tube