- Free electron laser
A free-electron laser, or FEL, is a
laser that shares the same optical properties as conventional lasers such as emitting abeam consisting of coherent electromagneticradiation which can reach high power, but which uses some very different operating principles to form the beam. Unlike gas,liquid , or solid-state lasers such asdiode laser s, in which electrons are excited in bound atomic or molecular states, FELs use arelativistic electron beam as the lasing medium which move freely through a magnetic structure, hence the term "free electron." [cite web |title=Duke University Free-Electron Laser Laboratory| url=http://www.fel.duke.edu/| accessdate=2007-12-21] The free-electron laser has the widestfrequency range of any laser type, and can be widely tunable, currently ranging inwavelength frommicrowave s, throughterahertz radiation andinfrared , to thevisible spectrum , toultraviolet , to softX-ray s.Beam creation
To create an FEL, a beam of
electrons is accelerated to relativistic speeds. The beam passes through an FEL oscillator in the form of a periodic, transversemagnetic field , produced by arrangingmagnet s with alternating poles within alaser cavity along the beam path. This array of magnets is sometimes called anundulator , or a "wiggler", because it forces the electrons in the beam to assume a sinusoidal path. The acceleration of the electrons along this path results in the release of aphoton (synchrotron radiation ). Since the electron motion is in phase with the field of the light already emitted, the fields add together (coherently). Instabilities in the electron beam, which result from the interactions of theoscillations of electrons in the undulators and the radiation they emit, leads to a bunching of the electrons which continue to radiate in phase with each other in contrast to conventional undulators where the electrons radiate independently. [Feldhaus et al., J. Phys. B: At. Mol. Opt. Phys. 38 (2005) S799–S819] The wavelength of the light emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic field strength of the undulators.Accelerators
Today, a free-electron laser requires the use of an electron accelerator with its associated shielding, as accelerated electrons are a radiation hazard. These accelerators are typically powered by
klystron s, which require a high voltage supply. Usually, the electron beam must be maintained in avacuum which requires the use of numerous pumps along the beam path. Free-electron lasers can achieve very high peak powers. Their tunability makes them highly desirable in several disciplines, including medicaldiagnosis and non-destructive testing.X-ray FELs
The lack of suitable
mirror s in the extremeultraviolet andx-ray regimes prevent the operation of an 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 utilise long undulators. The underlying principle of the intense pulses from the X-ray laser lies in the principle ofSelf-Amplified Stimulated-Emission (SASE), which leads to the microbunching of the electrons. Initially all electrons are evenly distributed but through the interaction of the oscillating electrons with the emitted radiation, the electrons drift into microbunches separated by a distance equal to one wavelength of the radiation. Through this arrangement, all the radiation emitted can reinforce itself perfectly whereby wave crests and wave troughs are always superimposed on one another in the best possible way. This is what leads to the high intensities and the laser-like properties.cite web |title=XFEL information webpages |url=http://xfelinfo.desy.de/en/start/2/index.html |accessdate=2007-12-21] Examples of facilities operating on the SASE FEL principle include the Free electron LASer in Hamburg (FLASH), the Linac Coherent Light Source (LCLS), currently being built at theStanford Linear Accelerator , and theEuropean x-ray free electron laser .One problem with SASE FELs is the lack of
temporal coherence due to a noisy startup process. To avoid this one can "seed" an FEL with alaser , produced by more conventional means, tuned to the resonance of the FEL. This results in coherentamplification of the input signal such that the output laser quality is characterized by the seed. This method becomes a problem atx-ray wavelength s because of the lack of conventional x-ray lasers.Medical applications
Research by Dr. Glenn Edwards and colleagues at Vanderbilt's FEL Center in 1994 found that soft tissues like skin, cornea, and grey matter could be cut, or ablated, using FEL wavelengths around 6.45 micrometres with minimal collateral damage to adjacent tissue. [ Glenn Edwards et al., Nature 371 (1994) 416-419] This led to further research and eventually surgeries on humans, the first ever using a free-electron laser. Starting in 1999, and using the Keck foundation funded FEL operating rooms at the Vanderbilt FEL Center, Dr. Michael Copeland and Dr. Pete Konrad of Vanderbilt performed three surgeries in which they resected mengioma brain tumors. [Glenn S. Edwards et al., Rev. Sci. Instrum. 74 (2003) 3207] Beginning in 2000, Dr. Karen Joos and Dr. Louise Mawn performed five surgeries involving the cutting of a window in the sheath of the optic nerve, to test the efficacy for optic nerve sheath fenestration. [Karen M. Joos et al., in Proc. of SPIE 4611 (2002) 81-85, eds. Manns, Soederberg, and Ho.] These eight surgeries went as expected with results consistent with the routine standard of care and with the added benefit of laser surgery and minimal collateral damage.
Since these successful results, there are several efforts to build small, clinical lasers tunable in the 6 to 7 micrometre range with pulse structure and energy to give minimal collateral damage in soft tissue. At Vanderbilt, there exists a Raman shifted system pumped by an Alexandrite laser.
At the 2006 annual meeting of the American Society for Laser Medicine and Surgery (ASLMS), Dr. Rox Anderson of the Wellman Laboratory of Photomedicine of
Harvard Medical School andMassachusetts General Hospital reported on the possible medical application of the free-electron laser in melting fats without harming the overlying skin. [cite web |title=BBC health |url=http://news.bbc.co.uk/1/hi/health/4895148.stm |accessdate=2007-12-21] It was reported that atinfrared wavelength s, water in tissue was heated by the laser, but atwavelength s corresponding to 915, 1210 and 1720 nm, subsurfacelipids 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, as well as targeting other lipids associated withcellulite and body fat as well as fatty plaques that form in artieries which can help treatatherosclerosis and heart disease. [cite web |title=Dr Rox Anderson treatment |url=http://www.physorg.com/news63806610.html |accessdate=2007-12-21]Military applications
FEL technology is considered by the
US Navy as a good candidate for an antimissiledirected-energy weapon . Significant progress is being made in increasing FEL power levels (already at 10 kW, as demonstrated at theJLab FEL) and it should be possible to build compact multi-megawatt class FEL lasers [cite web |title=Airbourne 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]ee also
*
TESLA particle accelerator Further reading
* Boscolo, "et al.", "Free-Electron Lasers and Masers on Curved Paths". Appl. Phys., (Germany), vol. 19, No. 1, pp. 46-51, May 1979.
* Deacon "et al.", "First Operation of a Free-Electron Laser". Phys. Rev. Lett., vol. 38, No. 16, Apr. 1977, pp. 892-894.
* 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.
* 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.
* Gover, "Collective and Single Electron Interactions of Electron Beams with Electromagnetic Waves and Free Electrons Lasers". App. Phys. 16 (1978), p. 121.
* "The FEL Program at Jefferson Lab" [http://www.jlab.org/fel]
* Citation | first = Charles | last = Brau | title = Free-Electron Lasers | place = Boston | publisher = Academic Press, Inc.
year = 1990References
External links
* [http://www.lightsources.org Lightsources.org]
* [http://sbfel3.ucsb.edu/ UCSB Free-Electron Laser]
* [http://www.nap.edu/books/NI000099/html/ Free-Electron Laser Open Book (National Academies Press)]
* [http://sbfel3.ucsb.edu/www/vl_fel.html The World Wide Web Virtual Library: Free-Electron Laser research and applications]
* [http://xfel.desy.de/ The European X-Ray Laser Project XFEL]
* [http://www.fz-rossendorf.de/pls/rois/Cms?pOid=10548&pNid=471 Electron beam transport system and diagnostics of the Dresden FEL]
* [http://www.rijnhuizen.nl/research/guthz/felix_felice The Free Electron Laser for Infrared eXperiments FELIX]
* [http://www.vanderbilt.edu/fel/ W. M. Keck Free Electron Laser Center]
* [http://www.jlab.org/fel Jefferson Lab's Free-Electron Laser Program]
* [http://www.newscientist.com/channel/mech-tech/mg18925351.300 Free-Electron Lasers: The Next Generation] by Davide CastelvecchiNew Scientist , January 21, 2006
* [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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