A Gyrotron is a high powered vacuum tube which generates millimeter-wave electromagnetic waves by bunching electrons with cyclotron motion in a strong magnetic field. Output frequencies range from about 20 to 250 GHz, covering wavelengths from microwave to the edge of the terahertz gap. Typical output powers range from tens of kilowatts to 1-2 megawatts. Gyrotrons can be designed for pulsed or continuous operation.
Principle of operation 
The gyrotron is a type of free electron maser (microwave amplification by stimulated emission of radiation). It has high power at millimeter wavelengths because its dimensions can be much larger than the wavelength, unlike conventional vacuum tubes, and it is not dependent on material properties, as are conventional masers. The bunching depends on a relativistic effect called the Cyclotron Resonance Maser instability. The electron speed in a gyrotron is slightly relativistic (comparable to but not close to the speed of light). This contrasts to the free electron laser (and xaser) that work on different principles and which electrons are highly relativistic.
Gyrotrons are used for many industrial and high technology heating applications. For example, gyrotrons are used in nuclear fusion research experiments to heat plasmas, and also in manufacturing industry as a rapid heating tool in processing glass, composites, and ceramics, as well as for annealing (solar and semiconductors). Military applications include the Active Denial System.
The gyrotron was invented in the Soviet Union. Present makers include Communications & Power Industries (USA), Gycom (Russia), Thales Group (EU),CEERI (INDIA), Toshiba (Japan) and Bridge12 Technologies, Inc.. System developers include Gyrotron Technology, Inc
See also 
- High-Magnetic-Field Research and Facilies (1979). Washington, D.C.: National Academy of Sciences. p. 51.
- The Gyrotron: A High Frequency Microwave Amplifier by A. Kupiszezwki.