Editing Free-electron laser (section)

== Construction==

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]].

===Infrared and terahertz FELs===
The [[Fritz Haber Institute of the Max Planck Society|Fritz Haber Institute]] in Berlin completed a mid-infrared and [[Terahertz spectroscopy and technology|terahertz]] FEL in 2013.<ref>{{Cite journal|last1=Schöllkopf|first1=Wieland|last2=Gewinner|first2=Sandy|last3=Junkes|first3=Heinz|last4=Paarmann|first4=Alexander|last5=von Helden|first5=Gert|last6=Bluem|first6=Hans P.|last7=Todd|first7=Alan M. M.|editor1-first=Sandra G|editor1-last=Biedron|date=201|title=The new IR and THz FEL facility at the Fritz Haber Institute in Berlin|journal=Advances in X-ray Free-Electron Lasers Instrumentation III|publisher=International Society for Optics and Photonics|volume=9512|pages=95121L|doi=10.1117/12.2182284|bibcode=2015SPIE.9512E..1LS|hdl=11858/00-001M-0000-0027-13DB-1|s2cid=55435812|hdl-access=free}}</ref><ref>{{Cite web|title=The FHI free-electron laser (FEL) facility|url=https://www.fhi.mpg.de/230029/FEL|website=Fritz Haber Institute of the Max Planck Society|language=en|access-date=2020-05-04}}</ref>

At [[Helmholtz-Zentrum Dresden-Rossendorf|Helmholtz-Zentrum Dresden - Rossendorf]] two terahertz and mid-infrared FEL-based sources are in operation. FELBE is an FEL equipped with a cavity with continuous pulsing with a repetition rate of 13 MHz, pulsing with 1 kHz by applying a pulse picker, and macrobunch operation with bunch length > 100 μs and macrobunch repetition rates ≤ 25 Hz. Pulse duration and pulse energy vary with wavelength and lie in the range from 1 - 25 ps and 100 nJ - few μJ, respectively. <ref>{{Cite web |title=FELBE |url=https://www.fels-of-europe.eu/fel_facilities/ir_facilities/felbe/ |access-date=2025-01-07 |website=www.fels-of-europe.eu}}</ref> The TELBE facility is based on a [[Superradiant emission|superradiant]] undulator offering THz pulses ranging from 0.1 THz to 2.5 THz at repetition rates up to 500 kHz.<ref>{{Cite web |title=TELBE - Helmholtz-Zentrum Dresden-Rossendorf, HZDR |url=https://www.hzdr.de/db/Cms?pOid=34100&pNid=2609 |access-date=2025-01-07 |website=www.hzdr.de |language=en}}</ref>

===X-ray FELs===
The lack of [[mirror]] materials that can reflect [[extreme ultraviolet]] and [[x-ray]]s means that X-ray free electron lasers (XFEL) need to work without a [[resonant cavity]]. Consequently, in an X-ray FEL (XFEL) the beam is produced by a single pass of radiation through the [[undulator]]. This requires that there be enough amplification over a single pass to produce an appropriate beam.

Hence, XFELs use long undulator sections that are tens or hundreds of meters long. This allows XFELs to produce the brightest X-ray pulses of any human-made x-ray source. The intense pulses from the X-ray laser lies in the principle of [[self-amplified spontaneous emission]] (SASE), which leads to microbunching. Initially all electrons are distributed evenly and emit only incoherent spontaneous radiation. Through the interaction of this radiation and the electrons' [[oscillation]]s, they drift into microbunches separated by a distance equal to one radiation wavelength. This interaction drives all electrons to begin emitting coherent radiation. Emitted radiation can reinforce itself perfectly whereby wave crests and wave troughs are optimally superimposed on one another. 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 |access-date=2007-12-21}}</ref> 

Examples of facilities operating on the SASE FEL principle include the:
* Free electron LASer in Hamburg ([[DESY#Particle accelerators and other facilities|FLASH]]) 
* [[Linac Coherent Light Source]] (LCLS) at the [[SLAC National Accelerator Laboratory]] 
* [[European x-ray free electron laser]] (EuXFEL) in Hamburg<ref>{{Cite journal|last=Doerr|first=Allison|date=November 2018|title=High-speed protein crystallography|journal=Nature Methods|volume=15|issue=11|pages=855|doi=10.1038/s41592-018-0205-x|pmid=30377367|doi-access=free}}</ref> 
* [[SPring-8]] Compact SASE Source (SCSS) in Japan
* [[SwissFEL]] at the [[Paul Scherrer Institute]] in Switzerland
* SACLA at the [[RIKEN]] Harima Institute in Japan
* PAL-XFEL (Pohang Accelerator Laboratory X-ray Free-Electron Laser) in Korea

In 2022, an upgrade to [[Stanford University]]'s [[SLAC National Accelerator Laboratory|Linac Coherent Light Source]] (LCLS-II) used temperatures around −271 °C to produce 10<sup>6</sup> pulses/second of near light-speed electrons, using superconducting niobium cavities.<ref>{{Cite web |last=Irving |first=Michael |date=2022-05-11 |title=World's most powerful X-ray laser now fires a million bursts per second |url=https://newatlas.com/science/lcls-ii-worlds-most-powerful-x-ray-laser/ |access-date=2022-05-16 |website=New Atlas |language=en-US}}</ref>

====Seeding and Self-seeding====
One problem with SASE FELs is the lack of [[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 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.

In late 2010, in Italy, the seeded-FEL source FERMI@Elettra<ref>{{cite web |url=http://www.elettra.trieste.it/lightsources/fermi.html |title=FERMI HomePage |publisher=Elettra.trieste.it |date=2013-10-24 |access-date=2014-02-17}}</ref> started commissioning, at the [[ELETTRA|Trieste Synchrotron Laboratory]]. FERMI@Elettra is a single-pass FEL user-facility covering the wavelength range from 100&nbsp;nm (12&nbsp;eV) to 10&nbsp;nm (124&nbsp;eV), located next to the third-generation synchrotron radiation facility ELETTRA in Trieste, Italy.

In 2001, at [[Brookhaven national laboratory]], a seeding technique called "High-Gain Harmonic-Generation" that works to X-ray wavelength has been developed.<ref>{{cite journal | doi=10.1103/PhysRevLett.86.5902 | title=Characterization of a High-Gain Harmonic-Generation Free-Electron Laser at Saturation | date=2001 | last1=Doyuran | first1=A. | last2=Babzien | first2=M. | last3=Shaftan | first3=T. | last4=Yu | first4=L. H. | last5=Dimauro | first5=L. F. | last6=Ben-Zvi | first6=I. | last7=Biedron | first7=S. G. | last8=Graves | first8=W. | last9=Johnson | first9=E. | last10=Krinsky | first10=S. | last11=Malone | first11=R. | last12=Pogorelsky | first12=I. | last13=Skaritka | first13=J. | last14=Rakowsky | first14=G. | last15=Wang | first15=X. J. | last16=Woodle | first16=M. | last17=Yakimenko | first17=V. | last18=Jagger | first18=J. | last19=Sajaev | first19=V. | last20=Vasserman | first20=I. | journal=Physical Review Letters | volume=86 | issue=26 | pages=5902–5905 | pmid=11415390 | bibcode=2001PhRvL..86.5902D }}</ref> The technique, which can be multiple-staged in an FEL to achieve increasingly shorter wavelengths, utilizes a longitudinal shift of the radiation relative to the electron bunch to avoid the reduced beam quality caused by a previous stage. This longitudinal staging along the beam is called "Fresh-Bunch".<ref>{{cite journal | doi=10.1016/0168-9002(92)91147-2 | title=The "fresh-bunch" technique in FELS | date=1992 | last1=Ben-Zvi | first1=I. | last2=Yang | first2=K.M. | last3=Yu | first3=L.H. | journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | volume=318 | issue=1–3 | pages=726–729 | bibcode=1992NIMPA.318..726B | url=https://digital.library.unt.edu/ark:/67531/metadc1058637/ }}</ref>  This technique was demonstrated at x-ray wavelength<ref>S. Di Mitri, et al, Double Stage Seeded FEL with Fresh Bunch Injection Technique at FERMI@ELETTRA, Proceedings of IPAC2013, Shanghai, China, ISBN 978-3-95450-122-9 pp. 1185-1186</ref><ref>{{cite journal | doi=10.1103/PhysRevAccelBeams.26.090702 | title=Wavelength control in high-gain harmonic generation seeded free-electron lasers | date=2023 | last1=Sottocorona | first1=F. | last2=Perosa | first2=G. | last3=Allaria | first3=E. | last4=Brynes | first4=A. | last5=Danailov | first5=M. B. | last6=De Ninno | first6=G. | last7=Di Mitri | first7=S. | last8=Garzella | first8=D. | last9=Penco | first9=G. | last10=Spezzani | first10=C. | last11=Giannessi | first11=L. | journal=Physical Review Accelerators and Beams | volume=26 | issue=9 | page=090702 | bibcode=2023PhRvS..26i0702S | doi-access=free }}</ref> at [[ELETTRA|Trieste Synchrotron Laboratory]].

A similar staging approach, named "Fresh-Slice", was demonstrated at the [[Paul Scherrer Institut]], also at X-ray wavelengths. In the Fresh Slice the short X-ray pulse produced at the first stage is moved to a fresh part of the electron bunch by a transverse tilt of the bunch.<ref>{{cite journal | doi=10.1103/PhysRevLett.132.035002 | title=Millijoule Femtosecond X-Ray Pulses from an Efficient Fresh-Slice Multistage Free-Electron Laser | date=2024 | last1=Wang | first1=Guanglei | last2=Dijkstal | first2=Philipp | last3=Reiche | first3=Sven | last4=Schnorr | first4=Kirsten | last5=Prat | first5=Eduard | journal=Physical Review Letters | volume=132 | issue=3 | page=035002 | pmid=38307082 | bibcode=2024PhRvL.132c5002W | url=https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A58311 }}</ref>   

In 2012, scientists working on the LCLS found an alternative solution to 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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