A gyrotron is a class of high-power linear-beam vacuum tubes that generates millimeter-wave electromagnetic waves by the cyclotron resonance of electrons in a strong magnetic field. Output frequencies range from about 20 to 527 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. The gyrotron was invented by Soviet scientists at NIRFI, based in Nizhny Novgorod, Russia. Principle The gyrotron is a type of free-electron maser that generates high-frequency electromagnetic radiation by stimulated cyclotron resonance of electrons moving through a strong magnetic field. It can produce high power at millimeter wavelengths because as a fast-wave device its dimensions can be much larger than the wavelength of the radiation. This is unlike conventional microwave vacuum tubes such as klystrons and magnetrons, in which the wavelength is determined by a single-mode resonant cavity, a slow-wave structure. Thus, as operating frequencies increase, the resonant cavity structures must decrease in size, which limits their power-handling capability. In the gyrotron, a hot filament in an electron gun (1) at one end of the tube emits an annular-shaped (hollow tubular) beam of electrons (6), which is accelerated by a high-voltage DC anode (10) and then travels through a large tubular resonant cavity structure (2) in a strong axial magnetic field, usually created by a superconducting magnet around the tube (8). The field causes the electrons to move helically in tight circles around the magnetic field lines as they travel lengthwise through the tube. At the position in the tube where the magnetic field reaches its maximum (2), the electrons radiate electromagnetic waves, parallel to the axis of the tube, at their cyclotron resonance frequency. The millimeter radiation forms standing waves in the tube, which acts as an open-ended resonant cavity, and is formed into a beam. The beam is converted by a mode converter (9) and reflected by mirrors (4) which direct it through a window (5) in the side of the tube into a microwave waveguide (7). The spent electron beam is absorbed by a collector electrode at the end of the tube (3). As in other linear-beam microwave tubes, the energy of the output electromagnetic waves comes from the kinetic energy of the electron beam, which is due to the accelerating anode voltage (10). In the region before the resonant cavity