High-power 140 GHz gyrotron for plasma heating in the Wendelstein 7-X fusion experiment, Germany. A gyrotron is a class of high-power linear-beam

vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...

s that generates millimeter-wave electromagnetic waves by the cyclotron resonance of

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

s in a strong

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

. Output

frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is e ...

range from about 20 to 527 GHz, covering wavelengths from

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

to the edge of the

terahertz gap Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequ ...

. Typical output powers range from tens of

kilowatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...

s to 1–2

megawatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...

s. Gyrotrons can be designed for pulsed or continuous operation. The gyrotron was invented by Soviet scientists at NIRFI, based in

Nizhny Novgorod Nizhny Novgorod ( ; rus, links=no, Нижний Новгород, a=Ru-Nizhny Novgorod.ogg, p=ˈnʲiʐnʲɪj ˈnovɡərət ), colloquially shortened to Nizhny, from the 13th to the 17th century Novgorod of the Lower Land, formerly known as Gork ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

Principle

The gyrotron is a type of free-electron

maser A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James ...

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

s such as

klystron A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequen ...

s and magnetrons, in which the wavelength is determined by a single-mode

resonant cavity A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonato ...

, a ''slow-wave'' structure. Thus, as operating frequencies increase, the resonant cavity structures must decrease in size, which limits their power-handling capability. A gyrotron (right) in cross-section (left). The electron path is shown in blue, and the generated microwave radiation in pink. In the gyrotron, a hot

filament The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar filament ...

in an

electron gun An electron gun (also called electron emitter) is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode-ray tubes (CRTs), used in nearly ...

(1) at one end of the tube emits an annular-shaped (hollow tubular) beam of

s (6), which is accelerated by a high-voltage DC

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...

(10) and then travels through a large tubular resonant cavity structure (2) in a strong axial

, usually created by a

superconducting magnet A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire has no electrical resistance and therefore can conduct much ...

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

, 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 A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...

(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 In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acce ...

of the electron beam, which is due to the accelerating anode voltage (10). 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 a

magnetic mirror A magnetic mirror, known as a magnetic trap (магнитный захват) in Russia and briefly as a pyrotron in the US, is a type of magnetic confinement device used in fusion power to trap high temperature plasma using magnetic fields. T ...

used in plasma confinement. 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 becomes

coherent Coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference * Coherence (units of measurement), a deriv ...

(synchronized) so they are all at the same point in their orbit at the same time. Therefore, they emit

coherent radiation A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

. The electron speed in a gyrotron is slightly relativistic (on the order of but not close to the speed of light). This contrasts to the free-electron laser (and

xaser An X-ray laser is a device that uses stimulated emission to generate or amplify electromagnetic radiation in the near X-ray or extreme ultraviolet region of the spectrum, that is, usually on the order of several tens of nanometers (nm) wavelength ...

) that work on different principles and whose electrons are highly relativistic.

Applications

Gyrotrons are used for many industrial and high-technology heating applications. For example, gyrotrons are used in

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

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 Active Denial System (ADS) is a non-lethal directed-energy weapon developed by the U.S. military, designed for area denial, perimeter security and crowd control. Informally, the weapon is also called the heat ray since it works by heating th ...

. In 2021 Quaise Energy announced the idea of using a gyrotron as a boring machine to drill a hole 20 kilometers in depth and use it to produce

geothermal energy Geothermal energy is the thermal energy in the Earth's crust which originates from the formation of the planet and from radioactive decay of materials in currently uncertain but possibly roughly equal proportions. The high temperature and pr ...

. The technique would use frequencies of 30 to 300 GHz and would transfer energy to a rock 10¹² more efficiently than using a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...

. Lasers would further be disrupted by the vaporized rock, which would affect the longer-wavelength much less. Drilling rates of 70 meters/hour appear to be possible with a 1-MW gyrotron that can achieve 100 percent efficiency.

Types

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 in the side of the tube. This requires a 45° mirror at the end of the cavity 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 in 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 gyrotron

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...

s 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 the klystron, so there is a series of gyrotrons which 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 a

tube. Has two microwave 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 a travelling 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 a backward wave oscillator (BWO). It generates oscillations traveling in an opposite direction to the electron beam, which are output at the upstream end of the tube. A gyro-twystron is an amplifier that functions analogously to a twystron, 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 which develops the amplified output signal. Like a TWT it has a wide bandwidth.

Manufacturers

The gyrotron was invented in the

Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...

. Present makers include Communications & Power Industries (USA), Gycom (Russia),

Thales Group Thales Group () is a French multinational company that designs, develops and manufactures electrical systems as well as devices and equipment for the aerospace, defence, transportation and security sectors. The company is headquartered in Pari ...

(EU),

Toshiba , commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. Its diversified products and services include power, industrial and social infrastructure systems, ...

(Japan, now Canon, Inc., also from Japan), and Bridge12 Technologies. System developers include Gyrotron Technology.

References

External links