Magnetron and Klystron requires resonant cavities to support the electromagnetic field that interacts with the electron beam. The travelling tube requires a slow wave structure. These structures have dimensions linearly proportional to the operating wavelengths. The consequence of having to reduce the dimensions, as the frequency increases, is that the available area for the electron beam decreases and the power o/p that can be achieved decreases rapidly. Thus at frequencies of 100 GHz and above conventional microwave tubes are not capable of producing power o/p in kW range.

Gyrotron tube developed does not rely on resonant cavities or slow wave structures. The electron gun generates a hollow electron beam, which is injected into a very strong magnetic field, provided by solenoid. The field causes the electrons to move helically in tight circles around the magnetic field lines as they travel lengthwise through the tube. The beam is then passed through a cavity where an interaction with an electromagnetic field occurs and RF power is generated. The beam and the RF power leave the cavity and travel through a cone-shaped expanding waveguide. The beam is collected on the walls of the waveguide and the RF propagates out of the gyrotron through the output waveguide and window. To make use of gyrotron as an amplifier cavity contains an i/p port along with the o/p port.