Free electron laser
A free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation 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 as diode lasers, in which electrons are excited in bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium which move freely through a magnetic structure, hence the term free electron. The free-electron laser has the widest frequency range of any laser type, and can be widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to ultraviolet, to soft X-rays.

# Beam creation
To create a FEL, a beam of electrons is accelerated to relativistic speeds. 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 assume 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).  Instabilities in the electron beam, which result 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 in contrast to conventional undulators where the electrons radiate independently.  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 klystrons, which require a high voltage supply.  Usually, the electron beam must be maintained in a vacuum 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 medical diagnosis and non-destructive testing.

# X-ray FELs
The lack of suitable mirrors in the extreme ultraviolet and x-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 of Self-Amplified Stimulated-Emission 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. 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 the Stanford Linear Accelerator, and the European 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 a laser, produced by more conventional means, tuned to the resonance of the FEL.  This results in coherent amplification of the input signal such that the output laser quality is characterized by the seed.  This method becomes a problem at x-ray wavelengths 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 microns 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 mengioma 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.
Since these successful results, there are several efforts to build small, clinical lasers tunable in the 6 to 7 micron 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 artieries which can help treat atherosclerosis and heart disease.

# Military applications
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 (already at 10 kW, as demonstrated at the JLab FEL) and it should be possible to build compact multi-megawatt class FEL lasers