In microwave tube to generate or to amplify the RF and microwave signal at larger power over a wider bandwidth non resonant periodic circuits or slow-wave structures are used. These structures are used to reduce the wave velocity in a certain direction so that the slow moving electrons of electron beam can interact with the fast moving electromagnetic wave. It is well known that the phase velocity of the EM wave in the waveguide is larger than velocity of the light in vacuum. Electron beam used in the vacuum tube can be accelerated only to the velocities that are far less than the velocity of the light. Due to this electrons of electron beam does not get sufficient time to interact with the wave. In order to reduce the velocity of the wave so that the electrons can get sufficient time to interact with wave, wave is passed through the slow wave-structure and thus the phase velocity of the wave is reduced.

Thus a slow wave structure is a non-resonating device used to reduce the phase velocity of the EM wave. Following are the examples of SLOW WAVE structures.