Agyrotron is a class of high-power linear-beamvacuum tubes that generatesmillimeter-wave electromagnetic waves by thecyclotron resonance ofelectrons in a strongmagnetic field. Outputfrequencies range from about 20 to 527GHz,[1][2] covering wavelengths frommicrowave to the edge of theterahertz gap. Typical outputpowers range from tens ofkilowatts to 1–2megawatts. Gyrotrons can be designed for pulsed or continuous operation. The gyrotron was invented bySoviet scientists[3] atNIRFI, based inNizhny Novgorod,Russia.
The gyrotron is a type of free-electronmaser that generates high-frequency electromagnetic radiation by stimulated cyclotron resonance of electrons moving through a strong magnetic field.[4][5] It can produce high power at millimeter wavelengths because, as afast-wave device, its dimensions can be much larger than the wavelength of the radiation. This is unlike conventional microwavevacuum tubes such asklystrons andmagnetrons, in which the wavelength is determined by a single-moderesonant cavity, aslow-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 hotfilament in anelectron gun (1) at one end of the tube emits an annular-shaped (hollow tubular) beam ofelectrons (6), which is accelerated by a high-voltageDCanode and then travels through a large tubular resonant cavity structure (2) in a strong axialmagnetic field, usually created by asuperconducting magnet around the tube (8). The field causes the electrons to movehelically 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 formsstanding waves in the tube, which acts as an open-endedresonant cavity, and is formed into a beam. The beam is converted by amode converter and reflected by mirrors (4), which direct it through a window (5) in the side of the tube into a microwavewaveguide. A collector electrode absorbs the spent electron beam at the end of the tube (3).[4][6]
As in other linear-beam microwave tubes, the energy of the output electromagnetic waves comes from thekinetic energy of the electron beam, which is due to the accelerating anode voltage. In the region before the resonant cavity where the magnetic field strength is increasing, it compresses the electron beam, converting the longitudinal drift velocity to transverse orbital velocity, in a process similar to that occurring in amagnetic mirror used inplasma confinement.[5] The orbital velocity of the electrons is 1.5 to 2 times their axial beam velocity. Due to the standing waves in the resonant cavity, the electrons become "bunched"; that is, their phase becomescoherent (synchronized), so they are all at the same point in their orbit at the same time. Therefore, they emitcoherent radiation.
The electron speed in a gyrotron is slightly relativistic (on the order of but not close to the speed of light). This contrasts to thefree-electron laser (andxaser) that work on different principles and whose electrons are highly relativistic.
Gyrotrons are used for many industrial and high-technology heating applications. For example, gyrotrons are used innuclear fusion research experiments to heatplasmas and also in the manufacturing industry as a rapid heating tool in processing glass, composites, and ceramics, as well as for annealing (solar and semiconductors). Military applications include theActive Denial System.
The output window of the tube from which the microwave beam emerges can be in two locations. In the transverse-output gyrotron, the beam exits through a window on the side of the tube. This requires a 45° mirror at the end of thecavity to reflect the microwave beam, positioned at one side so the electron beam misses it. In the axial-output gyrotron, the beam exits through a window at the end of the tube at the far end of the cylindrical collector electrode which collects the electrons.
The original gyrotron developed in 1964 was an oscillator, but since that time gyrotronamplifiers have been developed. The helical gyrotron electron beam can amplify an applied microwave signal similarly to the way a straight electron beam amplifies in classical microwave tubes such as theklystron, so there is a series of gyrotrons that function analogously to these tubes. Their advantage is that they can operate at much higher frequencies.The gyro-monotron (gyro-oscillator) is a single-cavity gyrotron that functions as an oscillator. A gyro-klystron is an amplifier that functions analogously to aklystron tube. Has twomicrowave cavities along the electron beam, an input cavity upstream to which the signal to be amplified is applied and an output cavity downstream from which the output is taken. A gyro-TWT is an amplifier that functions analogously to atravelling wave tube (TWT). It has a slow wave structure similar to a TWT paralleling the beam, with the input microwave signal applied to the upstream end and the amplified output signal taken from the downstream end. A gyro-BWO is an oscillator that functions analogously to abackward wave oscillator (BWO). It generates oscillations traveling in an opposite direction to the electron beam, which is output at the upstream end of the tube. A gyro-twystron is an amplifier that functions analogously to atwystron, a tube that combines a klystron and a TWT. Like a klystron, it has an input cavity at the upstream end followed by buncher cavities to bunch the electrons, which are followed by a TWT-type slow-wave structure that develops the amplified output signal. Like a TWT, it has a wide bandwidth.
The gyrotron was invented in theSoviet Union.[7] Present makers includeCommunications & Power Industries (USA),Gycom (Russia),Thales Group (EU),Kyoto Fusioneering (Japan),Toshiba (Japan, nowCanon, Inc.,[8] also from Japan), andBridge12 Technologies. System developers includeGyrotron Technology.