Free Electron Lasers
Study of characterstics of Free Electron Lasers
J.M.J. Madey invented the Free electron laser (FEL), in 1971. However, an important step in FEL development came in 1976 when Madey and his co-workers at Stanford University measured gain from an FEL configured as an amplifier at 10-μm wavelengths. This experiment, and the successful operation of the same FEL configured as an oscillator in 1977 at 3-μm wavelength, paved the way for a large interest in FEL research. These lasers are produced by the resonant interaction of a relativistic electron beam with a photon beam in an undulator or wiggler. Two important FEL attributes, tunability and design flexibility, were demonstrated by these two experiments at significantly different wavelengths using the same apparatus. These lasers offer wide range tunabilty and high brightness and are being considered in variety of applications. This 'synchrotron radiation' is a tool being used by physicists, chemists, materials scientists and biologists alike. They are tunable to different wavelengths and the light they emit is coherent. Rapid progress in FEL technology is opening new areas of research like exploring the behaviour of matter at the microscopic scale. FELs have a huge potential, and some US laboratories are exploring their application in industrial processing such as the modification of plastic surfaces, and also in surgery. These lasers are even being considered for deployment as a defence against guided missiles. The conventional lasers are based on the emissions arising from bound atoms and molecules. In FEL, on the other hand, the electrons are not bound but clustered into bunches and comprise of carefully controlled beam, which is accelerated close to the speed of light in an accelerator. The beam passes through an array of magnets, with alternating polarities called an undulator, which causes the electron beam to oscillate and emit synchrotron radiation in the process. The wavelength of the radiation can be tuned by altering the beam energy, or the strength of the magnetic fields. Further FELs can also be made to work at wavelengths not easily accessible by conventional lasers, and can operate continuously at high power. Although the principle behind the FEL was first explored in the 1970s, it is only in the past few years that scientist have started to exploit its potential at shorter wavelengths. There have been enormous advances in developing high-intensity electron sources, as well as superconducting accelerating devices and magnet technology - all necessary for the development of FELs. Today, there are more than 20 FEL facilities operating in the X - ray, infrared and visible-to-ultraviolet range, all over the world and another 15 - 20 are in various stages of development. The basic FEL system consists of an electron accelerator, an undulator or wiggler in which the electrons emit the syncrotron radiation, and an optical resonator. In FEL, a beam of electrons is accelerated to almost the speed of light. The beam passes through the FEL oscillator, a periodic transverse magnetic field produced by an arrangement of magnets with alternating poles within an optical cavity along the beam path. This array of magnets is 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 photons (synchrotron radiation). Since the electron motion is in phase with the field of the light already emitted, the fields add together coherently resulting in an exchange of electron energy with the electromagnetic field. This process is induced by the interaction of the electromagnetic radiation with the electrons. Since the radiation is faster than the electrons speeding along their path, the radiation overtakes the electrons flying ahead and interacts with them along the way, accelerating some of them and slowing others down. As a result of energy exchange, the electrons that gain energy begin to move ahead of the average electron, while the electrons that lose energy begin to fall behind the average. In the process, the beam of electrons gradually gets bunched on the scale of the radiation wavelength and this collective motion of bunches radiates powerful coherent synchrotron radiation. The emission rate for a perfectly bunched beam of electrons is proportional to the square of the number of electrons, whereas the emission rate for a beam of randomly positioned electrons is only proportional to the number of electrons.

Basic scheme of a free electron laserThe undulator gives a resonance condition between the electron bunch and the electromagnetic wave. The basic scheme of Free electron laser is shown in the adjoining figure. Over one undulator period, λw, the time difference between the electron bunch and the wave must correspond to the wavelength, λo, of the spontaneously emitted light, i.e. the longitudinal 'slippage' of the electrons relative to the light must equal, λo. Under resonance condition, the wavelength of the emitted radiation, λo, at the resonance depends on the electron energy and the magnitude and periodicity of the undulator and the magnetic field strength according to the relation
Undulator
γ is the relativistic factor and γmc2 is the energy of electrons. K is the undulator parameter, which is proportional to the magnetic field inside the undulator and is given as
Undulator
Where Bw is the undulator magnetic field strength in Tesla and λw is the undulator period length in centimeters.

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.

Giga watts peak powers have been demonstrated in pulses of pulse width of the order of femtoseconds.
Typical values of various parameters are given below

Some of the operating electron accelerators have following basic features:
Typical Parameters of Accelerators for Free Electron lasers
Peak Current Pulse Length Energy Wavelength Type of Accelerator
1 - 5 A Microseconds 1 - 10 MeV Infrared (100 μm to millimeter) Electrostatic
1 - 10 kA Nanoseconds 1 - 50 MeV Microns to centimeters Induction Linac
1-1000 A Picoseconds to microseconds 100 MeV - 10 GeV X-ray, UV, Visible (few nanometers to micron) Storage Ring
100 - 5000 A Femtosecond to picoseconds 10 MeV - 25 GeV X-ray to far infrared (nanometer to fraction of millimeter) RF Linac
The important properties of Free Electro Lasers are
Applications
References


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Updated: 6 April, 2015
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