Editing Gyrotron (section)

==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.<ref name="bridge12"></ref><ref name="Borie">{{cite journal|first=E.|last=Borie|title=Review of Gyrotron Theory|journal=EPJ Web of Conferences|volume=149|pages=04018|version=KfK 4898|date=c. 1990|url=http://bibliothek.fzk.de/zb/kfk-berichte/KFK4898.pdf|access-date=July 9, 2014|bibcode=2017EPJWC.14904018N|doi=10.1051/epjconf/201714904018|doi-access=free}}</ref> 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 tube]]s such as [[klystron]]s and [[magnetron]]s, 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.

[[file:Gyrotron.png|thumb|A gyrotron (right) in cross-section (left). The electron path is shown in blue, and the generated microwave radiation is in pink.]]

In the gyrotron, a hot [[electrical filament|filament]] in an [[electron gun]] (1) at one end of the tube emits an annular-shaped (hollow tubular) beam of [[electron]]s (6), which is accelerated by a high-voltage [[Direct current|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 [[helix|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 [[Transverse mode#Waveguides|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). A collector electrode absorbs the spent electron beam at the end of the tube (3).<ref name="bridge12">{{cite web |title=What is a Gyrotron? |url=https://www.bridge12.com/learn/what-is-a-gyrotron/ |website=Bridge12 Technologies |access-date=12 November 2022}}</ref><ref>{{cite web |title= General features of a gyrotron |url=https://www.epfl.ch/research/domains/swiss-plasma-center/research/tcv/research_tcv_heating/tcv-ecrh-eccd-system/tcv-gyrotrons |website=École polytechnique fédérale de Lausanne |access-date=12 November 2022}}</ref>

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 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]] used in [[plasma confinement]].<ref name="Borie" /> 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 radiation|coherent]] (synchronized), so they are all at the same point in their orbit at the same time. Therefore, they emit [[coherent 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 the [[free-electron laser]] (and [[xaser]]) that work on different principles and whose electrons are highly relativistic.