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| [[File:FELIX.jpg|265px|thumb|The free-electron laser ''FELIX'' at the FOM Institute for Plasma Physics Rijnhuizen, [[Nieuwegein]], The Netherlands.]]
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| A '''free-electron laser''' (FEL), is a type of [[laser]] that uses a [[relativistic electron beam]] that moves freely through a magnetic structure,<ref name="huang">{{Cite doi|10.1103/PhysRevSTAB.10.034801}}</ref> hence the term ''free electron'' as the [[active laser medium|lasing medium]].<ref>{{cite web |title=Duke University Free-Electron Laser Laboratory| url=http://www.fel.duke.edu/| accessdate=2007-12-21}}</ref>{{failed verification|date=October 2013}} The free-electron laser has the widest [[frequency]] range of any laser type, and can be widely tunable,<ref>[[F. J. Duarte]] (Ed.), ''Tunable Lasers Handbook'' (Academic, New York, 1995) Chapter 9.</ref> currently ranging in [[wavelength]] from [[microwave]]s, through [[terahertz radiation]] and [[infrared]], to the [[visible spectrum]], [[ultraviolet]], and [[X-ray]].<ref>{{cite web |title=New Era of Research Begins as World's First Hard X-ray Laser Achieves "First Light" |publisher=SLAC National Accelerator Laboratory |date=April 21, 2009 |accessdate=2013-11-06
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| | url=http://home.slac.stanford.edu/pressreleases/2009/20090421.htm}}</ref>
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| [[Image:FEL principle.png|400px|thumb|right|Schematic representation of an [[undulator]], at the core of a free-electron laser.]]
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| The term free-electron lasers was coined by [[John Madey]] in 1976 at [[Stanford University]].<ref>Hans Motz, W. Thon, R.N. Whitehurst, Experiments on radiation by fast electron beams, ''Journal of Applied Physics'', 24(7):826-833, 1953.</ref> The work emanates from research done by [[Hans Motz]] and his coworkers, who built an [[undulator]] at [[Stanford]] in 1953,<ref>{{Cite doi|10.1063.2F1.1700002}}</ref><ref>{{Cite doi|10.1063/1.1721389}}</ref> using the [[Wiggler (synchrotron)|wiggler]] magnetic configuration which is the heart of a free electron laser. Madey used a 43-MeV electron beam<ref>{{cite web|url=http://prl.aps.org/abstract/PRL/v38/i16/p892_1 |title=Phys. Rev. Lett. 38, 892 (1977): First Operation of a Free-Electron Laser |publisher=Prl.aps.org |date= |accessdate=2014-02-17}}</ref> and 5 m long wiggler to amplify a signal.
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| {{toclimit|3}}
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| == Beam creation ==
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| [[File:Undulator.FELIX.jpg|265px|thumb|The undulator of ''FELIX''.]]
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| To create an FEL, a beam of [[electron]]s is accelerated to almost the [[speed of light]]. The beam passes through an undulator, a side to side [[magnetic field]] produced by a periodic arrangement of [[magnet]]s with alternating [[dipole|poles]] across the beam path. The direction of the beam is called the longitudinal direction, while the direction across the beam path is called transverse. This array of magnets is commonly known as an [[undulator]] in the light source community, or a [[wiggler (synchrotron)|wiggler]],{{citation needed|date=October 2013}} because it forces the electrons in the beam to wiggle transversely along a [[sinusoid]]al path about the axis of the undulator.
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| The transverse acceleration of the electrons across this path results in the release of [[photons]] ([[synchrotron radiation]]), which are monochromatic but still incoherent,
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| because the electromagnetic waves from randomly distributed electrons interfere constructively and destructively in time, and the resulting radiation power scales linearly with the number of electrons. If an external laser is provided or if the synchrotron radiation becomes sufficiently strong, the transverse electric field of the radiation beam interacts with the transverse electron current created by the sinusoidal wiggling motion, causing some electrons to gain and others to lose energy to the optical field via the [[ponderomotive force]].
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| This energy modulation evolves into electron density (current) modulations with a period of one optical wavelength. The electrons are thus clumped, called ''microbunches'', separated by one optical wavelength along the axis. Whereas conventional undulators would cause the electrons to radiate independently, the radiation emitted by the bunched electrons are in phase, and the fields add together [[coherence (physics)|coherently]].
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| The FEL radiation intensity grows, causing additional microbunching of the electrons, which continue to radiate in phase with each other.<ref>{{cite doi|10.1088/0953-4075/38/9/023}}</ref> This process continues until the electrons are completely microbunched and the radiation reaches a saturated power several orders of magnitude higher than that of the undulator radiation.
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| The wavelength of the radiation emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic field strength of the undulators.
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| FELs are relativistic machines. The wavelength of the emitted radiation, <math>\lambda_r</math>, is given by <ref>{{Cite doi|10.1103/RevModPhys.74.685}}</ref>
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| : <math>\lambda_r = \frac{\lambda_u}{2 \gamma^2}(1+K^2)</math> ,
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| or when the wiggler strength parameter K, discussed below, is small
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| : <math>\lambda_r \propto \frac{\lambda_u}{2 \gamma^2}</math> ,
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| where <math>\lambda_u</math> is the undulator wavelength (the spatial period of the magnetic field), <math>\gamma</math> is the relativistic [[Lorentz factor]] and the proportionality constant depends on the undulator geometry and is of the order of 1.
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| This formula can be understood as a combination of two relativistic effects. Imagine you are sitting on an electron passing through the undulator. Due to [[Lorentz contraction]] the undulator is shortened by a <math>\gamma</math> factor and the electron experiences much shorter undulator wavelength <math>\lambda_u/\gamma</math>. However, the radiation emitted at this wavelength is observed in the laboratory frame of reference and the [[relativistic Doppler effect]] brings the second <math>\gamma</math> factor to the above formula. Rigorous derivation from Maxwell's equations gives the divisor of 2 and the proportionality constant. In an x-ray FEL the typical undulator wavelength of 1 cm is transformed to x-ray wavelengths on the order of 1 nm by <math>\gamma</math> ≈ 2000, i.e. the electrons have to travel with the speed of 0.9999998''c''.
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| === Wiggler strength parameter K ===
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| K, a [[dimensionless]] parameter, tells the wiggler strength as the relationship between the length of a period and the radius of bend,<ref>{{cite web |title=WIGGLER |author=Robert Soliday |date=2006-09-05
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| | publisher=Argon National laboratory
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| | accessdate=2013-10-18}}</ref>
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| : <math>K = \frac{\gamma \lambda_u}{2 \pi \rho }</math>
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| where <math>\rho</math> is the bending radius.
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| === Quantum effects ===
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| In most cases, the theory of [[classical electromagnetism]] adequately accounts for the behavior of free electron lasers.<ref name="fain">{{Cite doi|10.1364/JOSAB.4.000078}}</ref> For sufficiently short wavelengths, [[quantum mechanics|quantum effects]] of electron recoil and [[Electronic noise#Shot noise|shot noise]] may have to be considered.<ref>{{Cite doi|10.1063/1.34633}}</ref>
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| == Large facilities required ==
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| Free-electron lasers require the use of an electron [[particle accelerator|accelerator]] with its associated shielding, as accelerated electrons can be a radiation hazard if not properly contained. These accelerators are typically powered by [[klystron]]s, which require a high voltage supply. The electron beam must be maintained in a [[vacuum]] which requires the use of numerous [[vacuum pump]]s along the beam path. While this equipment is bulky and expensive, free-electron lasers can achieve very high peak powers, and the tunability of FELs makes them highly desirable in many disciplines, including chemistry, structure determination of molecules in biology, [[medical diagnosis]], and [[nondestructive testing]].
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| == X-ray laser without mirrors ==
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| The lack of suitable [[mirror]]s in the extreme [[ultraviolet]] and [[x-ray]] regimes prevents the operation of a FEL oscillator; consequently, there must be suitable amplification over a single pass of the electron beam through the undulator to make the FEL worthwhile. X-ray free electron lasers use long [[undulator]]s. The underlying principle of the intense pulses from the X-ray laser lies in the principle of [[self-amplified stimulated emission]] (SASE), which leads to the microbunching. Initially all electrons are distributed evenly and they emit incoherent spontaneous radiation only. Through the interaction of this radiation and the electrons' [[oscillation]]s, they drift into microbunches separated by a distance equal to one radiation wavelength. Through this interaction, all electrons begin emitting coherent radiation in phase. All emitted radiation can reinforce itself perfectly whereby wave crests and wave troughs are always superimposed on one another in the best possible way. This results in an exponential increase of emitted radiation power, leading to high beam intensities and laser-like properties.<ref name=XFEL>{{cite web |title=XFEL information webpages |url=http://xfelinfo.desy.de/en/start/2/index.html |accessdate=2007-12-21}}</ref> Examples of facilities operating on the SASE FEL principle include the Free electron LASer ([[DESY#Particle Accelerators, Facilities and Experiments at DESY|FLASH]]) in Hamburg, the [[Linac Coherent Light Source]] (LCLS) at the [[SLAC National Accelerator Laboratory]], the [[European x-ray free electron laser]] (XFEL) in Hamburg, the [[SPring-8]] Compact SASE Source (SCSS), the [[SwissFEL]] at the [[Paul Scherrer Institute]] (Switzerland) and, as of 2011, the SACLA at the [[RIKEN]] Harima Institute in Japan.
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| == Self seeding ==
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| One problem with SASE FELs is the lack of [[time|temporal]] coherence due to a [[shot noise|noisy]] startup process. To avoid this, one can "seed" an FEL with a [[laser]] tuned to the resonance of the FEL. Such a temporally coherent seed can be produced by more conventional means, such as by [[high-harmonic generation]] (HHG) using an optical laser pulse. This results in coherent [[:wikt:amplification|amplification]] of the input signal; in effect, the output laser quality is characterized by the seed. While HHG seeds are available at [[wavelength]]s down to the extreme ultraviolet, seeding is not feasible at [[x-ray]] wavelengths due to the lack of conventional x-ray lasers.
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| In late 2010, in Italy, the seeded-FEL source FERMI@Elettra <ref>{{cite web|url=http://www.elettra.trieste.it/FERMI/ |title=FERMI / HomePage |publisher=Elettra.trieste.it |date=2013-10-24 |accessdate=2014-02-17}}</ref> started commissioning, at the Sincrotrone Trieste Laboratory. FERMI@Elettra is a single-pass FEL user-facility covering the wavelength range from 100 nm (12 eV) to 10 nm (124 eV), located next to the third-generation synchrotron radiation facility ELETTRA in Trieste, Italy. The advent of [[femtosecond]] lasers has revolutionized many areas of science from [[solid state physics]] to [[biology]].
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| In 2012, scientists working on the LCLS overcame the seeding limitation for x-ray wavelengths by self-seeding the laser with its own beam after being filtered through a diamond [[monochromator]]. The resulting intensity and monochromaticity of the beam were unprecedented and allowed new experiments to be conducted involving manipulating atoms and imaging molecules. Other labs around the world are incorporating the technique into their equipment.<ref name="nature">
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| {{Cite doi|10.1038/nphoton.2012.180}}</ref><ref>{{cite web
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| | title="Self-seeding" promises to speed discoveries, add new scientific capabilities|date=August 13, 2012 |publisher=SLAC National Accelerator Laboratory
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| | url=https://news.slac.stanford.edu/press-release/worlds-most-powerful-x-ray-laser-beam-refined-scalpel-precision |accessdate=2013-11-06}}</ref>
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| == Applications ==
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| === Medical ===
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| ==== Surgery ====
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| Research by Glenn Edwards and colleagues at [[Vanderbilt University]]'s FEL Center in 1994 found that soft tissues including skin, [[cornea]], and brain tissue could be cut, or [[ablation|ablated]], using [[infrared]] FEL wavelengths around 6.45 micrometres with minimal collateral damage to adjacent tissue.<ref>{{cite doi|10.1038/371416a0}}</ref><ref>{{cite web |title=Laser light from Free-Electron Laser used for first time in human surgery |url=http://www.vanderbilt.edu/News/register/Jan10_00/story5.html |accessdate=2010-11-06}}</ref> This led to surgeries on humans, the first ever using a free-electron laser. Starting in 1999, Copeland and Konrad performed three surgeries in which they resected [[meningioma]] [[brain tumor]]s.<ref>Glenn S. Edwards et al., Rev. Sci. Instrum. 74 (2003) 3207</ref> Beginning in 2000, Joos and Mawn performed five surgeries that cut a window in the sheath of the [[optic nerve]], to test the efficacy for optic nerve sheath fenestration.<ref>{{cite doi|10.1117/12.596603}}</ref> These eight surgeries produced results consistent with the [[standard of care]] and with the added benefit of minimal collateral damage. A review of FELs for medical uses is given in the 1st edition of Tunable Laser Applications.<ref name="Duarte2010">{{cite book|author=F. J. Duarte|title=Tunable Laser Applications, Second Edition|url=http://books.google.com/books?id=FCDPZ7e0PEgC|date=12 December 2010|publisher=CRC Press|isbn=978-1-4200-6058-4 |chapter=6}}</ref>
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| </ref>
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| ==== Fat removal ====
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| Several small, clinical lasers tunable in the 6 to 7 micrometre range with pulse structure and energy to give minimal collateral damage in soft tissue were created.{{Citation needed|date=November 2010}} At Vanderbilt, there exists a Raman shifted system pumped by an Alexandrite laser.<ref>{{cite web |title=Efficiency and Plume Dynamics for Mid-IR Laser Ablation of Cornea |url=http://meetings.aps.org/link/BAPS.2009.MAR.T27.6 |accessdate=2010-11-06 |date=2009-03-18}}</ref>
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| [[R. Rox Anderson|Rox Anderson]] proposed the medical application of the free-electron laser in melting fats without harming the overlying skin.<ref>{{cite news |title=BBC health |url=http://news.bbc.co.uk/1/hi/health/4895148.stm |accessdate=2007-12-21 | date=2006-04-10 |work=BBC News}}</ref> At [[infrared]] [[wavelength]]s, water in tissue was heated by the laser, but at wavelengths corresponding to 915, 1210 and 1720 [[nanometer|nm]], subsurface [[lipid]]s were differentially heated more strongly than water. The possible applications of this selective photothermolysis (heating tissues using light) include the selective destruction of sebum lipids to treat [[acne vulgaris|acne]], as well as targeting other lipids associated with [[cellulite]] and body fat as well as fatty plaques that form in arteries which can help treat [[atherosclerosis]] and [[heart disease]].<ref>{{cite web |title=Dr Rox Anderson treatment |url=http://www.physorg.com/news63806610.html |accessdate=2007-12-21}}</ref>
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| === Biology ===
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| Exceptionally bright and fast X-rays can image proteins using a sheet just one molecule thick. This technique allows first-time imaging of proteins that do not stack in a way that allows imaging by conventional techniques, some 25% of the total number of proteins. Resolutions of .8 nm have been achieved with pulse durations of 30 [[femtosecond]]s. To get a clear view resolution of .1-.3 nm is required. The short pulse durations prevented the lasers from destroying the molecules. The bright, fast X-rays were produced at the [[Linac Coherent Light Source]] at SLAC. As of 2014 LCLS was the world’s most powerful X-ray FEL.<ref>{{cite web|url=http://www.kurzweilai.net/super-bright-fast-x-ray-free-electron-lasers-can-now-image-single-layer-of-proteins |title=Super-bright, fast X-ray free-electron lasers can now image single layer of proteins |doi=10.1107/S2052252514001444 |publisher=KurzweilAI |date= |accessdate=2014-02-17}}</ref>
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| === Military ===
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| FEL technology is being evaluated by the [[US Navy]] as a candidate for an [[antiaircraft]] and missile [[directed-energy weapon]]. The [[Thomas Jefferson National Accelerator Facility]]'s FEL has demonstrated over 14 kW power output.<ref>{{cite web | title=Jefferson Lab FEL |url=http://www.jlab.org/fel/ |accessdate=2009-06-08}}</ref> Compact multi-megawatt class FEL weapons are undergoing research.<ref>{{cite web |title=Airborne megawatt class free-electron laser for defense and security |url=http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=841301 |accessdate=2007-12-21}}</ref> On June 9, 2009 the [[Office of Naval Research]] announced it had awarded [[Raytheon]] a contract to develop a 100 kW experimental FEL.<ref>{{cite web |title=Raytheon Awarded Contract for Office of Naval Research's Free Electron Laser Program |url=http://news.prnewswire.com/ViewContent.aspx?ACCT=109&STORY=/www/story/06-09-2009/0005040843&EDATE= |accessdate=2009-06-12}}</ref> On March 18, 2010 Boeing Directed Energy Systems announced the completion of an initial design for U.S. Naval use.<ref>{{cite web |title=Boeing Completes Preliminary Design of Free Electron Laser Weapon System |url=http://boeing.mediaroom.com/index.php?s=43&item=1126 |accessdate=2010-03-29}}</ref> A prototype FEL system was demonstrated, with a full-power prototype scheduled by 2018.<ref>{{cite news | url=http://www.foxnews.com/scitech/2011/01/20/raygun-breakthrough-revolutionize-naval-power/?test=faces | title=Breakthrough Laser Could Revolutionize Navy's Weaponry | accessdate=2011-01-22 | publisher=Fox News | date=2011-01-20}}</ref>
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| == See also ==
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| * [[Bremsstrahlung]]
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| * [[Cyclotron radiation]]
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| * [[Electron wake]]
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| * [[Gyrotron]]
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| * [[International Linear Collider]]
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| * [[Synchrotron radiation]]
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| == References ==
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| {{reflist|2}}
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| == Further reading ==
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| * Madey, John, "Stimulated emission of bremsstrahlung in a periodic magnetic field". J. Appl. Phys. 42, 1906 (1971)
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| * Madey, John, Stimulated emission of radiation in periodically deflected electron beam, US Patent 38 22 410,1974
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| * Boscolo, ''et al.'', "''Free-Electron Lasers and Masers on Curved Paths''". Appl. Phys., (Germany), vol. 19, No. 1, pp. 46–51, May 1979.
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| * Deacon ''et al.'', "''First Operation of a Free-Electron Laser''". Phys. Rev. Lett., vol. 38, No. 16, Apr. 1977, pp. 892–894.
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| * Elias, ''et al.'', "''Observation of Stimulated Emission of Radiation by Relativistic Electrons in a Spatially Periodic Transverse Magnetic Field''", Phys. Rev. Lett., 36 (13), 1976, p. 717.
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| * Gover, "''Operation Regimes of Cerenkov-Smith-Purcell Free Electron Lasers and T. W. Amplifiers''". Optics Communications, vol. 26, No. 3, Sep. 1978, pp. 375–379.
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| * Gover, "''Collective and Single Electron Interactions of Electron Beams with Electromagnetic Waves and Free Electrons Lasers''". App. Phys. 16 (1978), p. 121.
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| * "''The FEL Program at Jefferson Lab''" [http://www.jlab.org/fel]
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| * {{Cite journal | first = Charles | last = Brau | title = Free-Electron Lasers | place = Boston | publisher = Academic Press, Inc.
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| | year = 1990 | postscript = <!-- None --> }}
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| * Paolo Luchini, Hans Motz, ''Undulators and Free-electron Lasers'', Oxford University Press, 1990.
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| == External links ==
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| * [http://www.lightsources.org Lightsources.org]
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| * [http://www.elettra.trieste.it/lightsources/fermi.html FERMI], the new FEL at the [[ELETTRA]] synchrotron in Triest
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| * [http://sbfel3.ucsb.edu/ UCSB Free-Electron Laser]
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| * [http://www.nap.edu/books/NI000099/html/ Free-Electron Laser Open Book (National Academies Press)]
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| * [http://sbfel3.ucsb.edu/www/vl_fel.html The World Wide Web Virtual Library: Free-Electron Laser research and applications]
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| * [http://www.xfel.eu/ European XFEL]
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| * [http://fel.web.psi.ch/ PSI SwissFEL]
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| * [http://www-xfel.spring8.or.jp/ SPring-8 Compact SASE Source]
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| * [http://www.fz-rossendorf.de/pls/rois/Cms?pOid=10548&pNid=471 Electron beam transport system and diagnostics of the Dresden FEL]
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| * [http://www.rijnhuizen.nl/research/guthz/felix_felice The Free Electron Laser for Infrared eXperiments FELIX]
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| * [http://www.vanderbilt.edu/fel/ W. M. Keck Free Electron Laser Center]
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| * [http://www.jlab.org/fel Jefferson Lab's Free-Electron Laser Program]
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| * [http://www.newscientist.com/channel/mech-tech/mg18925351.300 Free-Electron Lasers: The Next Generation] by Davide Castelvecchi [[New Scientist]], January 21, 2006
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| * [http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=841301 Airborne megawatt class free-electron laser for defense and security]
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| * [http://www.elettra.trieste.it/FERMI/ FERMI@Elettra Free-Electron Laser Project]
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| * [http://www.cfel.de Center for Free-Electron Laser Science (CFEL)]
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| {{DEFAULTSORT:Free-Electron Laser}}
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| [[Category:Electron beam]]
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| [[Category:Laser types]]
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| [[Category:Medical equipment]]
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| [[Category:Terahertz technology]]
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| [[Category:Accelerator physics]]
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