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Free Electron LaserA Free Electron Laser, or FEL, generates tunable, coherent, high power radiation, currently ranging wavelengths from millimeters to visible. While an FEL laser beam shares the same optical properties as conventional lasers such as coherent radiation, the operation of an FEL is quite different. Unlike gas or diode lasers which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free-electron. Free electron lasers can be used to generate terahertz radiation. To create an FEL, a beam of electrons is accelerated to relativistic speeds. The beam passes through a periodic, transverse magnetic field. This field is produced by arranging magnets with alternating poles along the beam path. This array of magnets is sometimes called 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 (bremsstrahlung). Viewed relativistically in the rest frame of the electron, the magnetic field can be treated as if it was a virtual photon. The collision of the electron with this virtual photon in the creates an actual photon. Mirrors capture the released photons to generate resonant gain. Adjusting either the beam energy (speed/energy of the electrons) or the field strength tunes the wavelength easily and rapidly over a wide range. Since the photons emited are related to the electron beam and magnetic field strength, an FEL can be tuned, i.e. the frequency or color can be controlled. The free electron laser requires an electron accelerator with its associated shielding, as accelerated electrons are a radiation hazard. Additionally, the high energy electrons are generated by klystron or some accelerator, which requires a large voltage supply. The electron beam usually 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 make them highly desirable in several disciplines including medical diagnosis and non-destructive testing. References
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