Free electron laser schematic of operation
To create an FEL, a beam of electrons is accelerated to almost light speed (technically known as relativistic speed). The beam passes through an FEL oscillator in the form of a periodic, transverse magnetic field, produced by arranging magnets with alternating poles within a laser cavity along the beam path. This array of magnets is sometimes called an undulator, or a "wiggler", because it forces the electrons in the beam to follow a sinusoidal path. The acceleration of the electrons along this path results in the release of a photon (synchrotron radiation). Since the electron motion is in phase with the field of the light already emitted, the fields add together coherently. Whereas conventional undulators would cause the electrons to radiate independently, instabilities in the electron beam resulting from the interactions of the oscillations 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. 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.
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 klystrons, which require a high voltage supply. The electron beam must be maintained in a vacuum which requires the use of numerous pumps 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 several disciplines, including medical diagnosis and non-destructive testing.
The lack of suitable mirrors in the extreme ultraviolet and x-ray regimes prevent 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 utilise long undulators. The underlying principle of the intense pulses from the X-ray laser lies in the principle of Self-Amplified Spontaneous Emission (SASE), which leads to the microbunching of the electrons. Initially all electrons are distributed evenly and they only emit incoherent spontaneous radiation. Through the interaction of this radiation and the electrons' oscillations, they drift into microbunches separated by a distance equal to one radiation wavelength. Through this interaction, all electrons begin emitting coherent radiation in phase. In other words, 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. Examples of facilities operating on the SASE FEL principle include the Free electron LASer in Hamburg (FLASH), the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, the European x-ray free electron laser, the SPring-8 Compact SASE Source (SCSS) and the PSI SwissFEL.
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 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 wavelengths down to the extreme ultraviolet, seeding is currently not feasible at x-ray wavelengths due to the lack of conventional x-ray lasers.
Research by Dr. Glenn Edwards and colleagues at Vanderbilt University's FEL Center in 1994 found that soft tissues like skin, cornea, and brain tissue could be cut, or ablated, using FEL wavelengths around 6.45 micrometres with minimal collateral damage to adjacent tissue. 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 meningioma brain tumors. 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. 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. A review of FELs for medical uses is given in the 1st edition of Tunable Laser Applications.
Since these successful results, there have been 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 and Massachusetts General Hospital reported on the possible medical application of the free-electron laser in melting fats without harming the overlying skin. It was reported that at infrared wavelengths, water in tissue was heated by the laser, but at wavelengths corresponding to 915, 1210 and 1720 nm, subsurface lipids 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 with cellulite and body fat as well as fatty plaques that form in arteries which can help treat atherosclerosis and heart disease.
FEL technology is considered by the US Navy as a good candidate for an antimissile directed-energy weapon. Significant progress is being made in increasing FEL power levels (the Thomas Jefferson National Accelerator Facility's FEL has demonstrated greater than 14kW ) and it should be possible to build compact multi-megawatt class FEL weapons. On June 9, 2009 the Office of Naval Research announced it had awarded Raytheon a contract to develop a 100kW experimental FEL.
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The European X-Ray Free-Electron Laser Project XFEL
SPring-8 Compact SASE Source
Electron beam transport system and diagnostics of the Dresden FEL
The Free Electron Laser for Infrared eXperiments FELIX
W. M. Keck Free Electron Laser Center
Jefferson Lab's Free-Electron Laser Program
Free-Electron Lasers: The Next Generation by Davide Castelvecchi New Scientist, January 21, 2006
Airborne megawatt class free-electron laser for defense and security
FERMI@Elettra Free-Electron Laser Project
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