Free Electron Lasers

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.

The undulator gives a resonance condition between the electron bunch and the electromagnetic wave. The basic scheme of Free electron laser is shown in the figure below. Over one undulator period, λ_w, the time difference between the electron bunch and the wave must correspond to the wavelength, λ_o, of the spontaneously emitted light, i.e. the longitudinal 'slippage' of the electrons relative to the light must equal, λ_o. Under resonance condition, the wavelength of the emitted radiation, λ_o, 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

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

Where B_w is the undulator magnetic field strength in Tesla and λ_w 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

• Peak Magnetic field: few kilogauss
• Wavelength: few Angstroms to 100 mm
• Number of undulator periods: 100
• Undulator period λ_w: 2 - 10 cm
• Length of Undulator: 10 meters
• Electron beam energy: Few MeV to Several GeV
• Electron beam radius: About 1mm
• Electron beam pulse: nanoseconds to femtoseconds
• Efficiency: up to 40% at longer wavelengths but less at shorter wavelengths
• Photon beam size (FWHM) ~ 100 μm
• Photon beam divergence (FWHM) < μrad
• Pulse duration (FWHM) ~ 100 fs
• Min. pulse separation ~ 90-100 ns
• Max. Number of pulses per train ~ 11500
• Repetition rate: 5 Hz
• Number of photons per pulse: 1.8 x 10¹²
• Excellent beam quality M² < 1.1
• Tunability 10 GHz - 1Å

Some of the operating electron accelerators have following basic features:

Typical Parameters of Accelerators for Free Electron lasers

• Radiation from a Free Electron Laser (FEL) has many common features in common with radiation from a conventional optical laser, such as high power; narrow bandwidth and diffraction limited beam propagation. One of the main differences between the two lasers is the gain medium: In a conventional LASER, the amplification comes from the stimulated emission of electrons bound to atoms, either in a crystal, liquid dye or a gas, whereas the amplification medium of the FEL are "free" (unbound) electrons. The free electrons have been stripped from atoms in an electron gun and are then accelerated to relativistic velocities.
• One can have an idea about the velocity of electrons from the relation:
  
  Where ν is the velocity of electrons, c is the velocity of light and γ is a factor related to energy of electrons as:
  
  The electrons having energy of one MeV will have velocity of about 86 % of the velocity of light. Similarly, electrons with energy of 10 Mev and 100 MeV will have velocity of the order of 99.9 % and 99.999% of the velocity of light.
• Ordinary lasers, however, operate at a fixed frequency. Though efforts have been made to have large number of wavelengths by tuning or having second or third harmonic generation, but still the choice is limited. That limits their usefulness. However, the FELs are ideal for exploring the unknown regions in the spectrum because these are tunable over a broad range of the spectrum. That enables these lasers more useful for material, medical and military applications.
• FELs in principle can produce radiation at any wavelength. However, the system becomes complicated, as the required wavelength is on the lower side for example UV or X-rays.
• Since the FEL uses a single gain medium, the relativistic electron beam, and because the resonant condition can be easily tuned by changing either the electron beam energy or the magnetic field strength, Tunability by a factor of about 10 i.e ten times the tunable frequency range has already been demonstrated with the same accelerator and undulator.
• As waste energy is carried away at nearly the speed of light and because high optical fields cannot damage the lasing medium, FELs can produce very high peak powers. Gigawatt peak powers have been demonstrated.
• The pulse structure of the radiation follows the pulse structure of the electron beam, the mature RF technology of linear accelerators can be used to manipulate and control the FEL pulse structure.
• FELs easily achieve properties such as a single transverse mode, high spatial and temporal coherence, and flexible polarization properties, comparable to conventional lasers,
• Smaller the undulator period, lesser energy is required for generation of a given wavelength.
• The linewidth of the laser radiation is determined by the number Nw of undulator periods; larger the number of periods, narrower is the line width. The linewidth is given as
  Δλ / λ = 1 / 2 N_w
• The maximum FEL efficiency, defined as ratio of the laser intensity to the initial beam kinetic energy, is of the order of 1 / 2 N_w.
• The FEL can be operated in three different modes:
  • Oscillator: In the case of the FEL oscillator, the optical pulses are bouncing between the cavity mirrors of an open optical resonator. An optical cavity has many advantages: it requires less gain per pass, simplifying the undulator, and it facilitates the production of narrow bandwidth output radiation. However, it is difficult to utilize optical cavities at short wavelengths because one requires high quality mirrors resistant to radiation damage.
  • Amplifier: In the case of the FEL amplifier, there is no optical resonator; a seed laser sends optical pulses synchronized to overlap the electron pulses as they enter the undulator. The FEL process is initiated by an external coherent signal. The external coherent signal is amplified by the FEL, known as direct seeding. The output power preserves the properties of the input signal (coherence, wavelength). However, the tunability range is limited by the availability of coherent sources.
  • Self-amplified spontaneous radiation (SASE): The process is initiated by shot noise and both temporal and spectral properties are affected by that They are tunable: the radiation wavelength can be continuously varied by changing the electron-beam energy and/or the undulator parameter K in the resonant condition The system is completely tunable and it is possible to reach very high peak power. They allow generating high intensity, short pulse radiation in the spectral region from deep ultraviolet down to hard x-ray wavelengths. Single pass FELs are being considered as the next generation light sources
• For longer wavelengths, the emission can be amplified by placing mirrors beyond the ends of the undulator; the mirrors bounce the light beam back and forth to increase interaction with the electron beam. But, for shorter wavelengths, there are no suitable mirrors and so to achieve enough gain, an intense electron beam is sent down a much longer undulator with several thousand alternating magnets.
• One of the emerging areas in FEL is High-gain Harmonic Generation (HGHG) FEL laser. A simple HGHG FEL scheme has three components: one undulator used as the modulator, dispersion section, and a second undulator used as the radiator. A seed laser, together with an electron beam, is introduced into the modulator. In the modulator, the seed laser interacts with the e-beam, and a small energy modulation is formed in the e-beam. The energy-modulated e-beam passes through the dispersion section, where the energy modulation in the e-beam is converted into spatial modulation. Because the high-peak- power, high quality seed laser dominates the spontaneous emission from the electron beam itself, the phase information of the seed laser is preserved in the spatial modulation in the electron beam. A large number of harmonics exist in such spatially modulated e-beam, which then enters the radiator. The radiator is designed to be resonant to one of the harmonics of the seed laser frequency. Once the spatially modulated e-beam enters the radiator, rapid coherent emission at this resonant harmonic is produced, and then this harmonic is further amplified exponentially until saturation. Typically with an input CO₂ seed laser power of 0.7 MW at 10.6 μm, the output HGHG FEL power of about 35 MW at 5.3 μm has been obtained.
• Another technique to produce harmonics is through Self-Induced Harmonic Generation (SIHG). This is a promising technique to obtain shorter wavelengths in, for an example, self-amplified spontaneous emission (SASE) FELs, since not only the fundamental but also the higher harmonics achieve saturation nearly simultaneously. This occurs in all single-pass, high-gain, free-electron lasers configured with planar undulators. The sinusoidal electron beam traversal through these undulators naturally forces the odd harmonics to be favored.
• These harmonics may either be exploited as a radiation source or may serve as a seed laser for FEL operation at shorter wavelengths operating in the VUV-X region of the spectrum.
• The FEL is increasingly gaining importance as directed energy weapon of choice for the 21st century. Main features of FEL are these are all electric with an "infinite magazine", and a continuously tunable wavelength. This is because the basis of the FEL is only electrons and a magnetic field. Other lasers use some type of active medium, which would likely require periodic replacement or replenishment. Other major advantage of the FEL is its readily adjustable wavelength. Another advantage of the FEL is its capability for relatively high efficiency. The wallplug efficiency, which is defined as the ratio of the average radiation power output to the electric power input in case of an FEL, can be as high as 40%. The single-pass extraction efficiency, β is the fraction of the electron beam power that is converted to optical power over a single pass. The typical extraction efficiency is estimated to be about 5%.
• Though Army and Air force are concentrating on Solid State lasers and Chemical Lasers respectively, Navy however is pursuing mainly Free Electron Lasers. As per the reported data, Free electron lasers capable of delivering 14 kW have already been developed with immediate goals of developing 100 kW and upgrading later on to a Megawatt level. Magazine depth is an important issue The FEL magazine is much deeper as compared to that of other lasers.

Applications

• Material science for micro machining, metal surface processing, polymer surface processing, electronic material processing, Nanotube synthesis
• Atmospheric research,
• Isotope separation
• Spectroscopic tools for imaging, and to probe dynamical processes in real time on timescales down to tens of femtoseconds.
• The development of X-ray Free electro lasers will help researchers to take snapshots of chemical bonds being made and broken, and to look at detailed physical processes such as planes of atoms sliding over one another.
• To study subtle molecular changes in biological systems
• Medical applications like surgery where the beam needs enough energy to vaporize soft tissue and bone. Some of these applications may be based on the clean cutting of soft tissue. Other uses may include welding tissue to assist in wound healing, repairing nerves, reattaching retinas or monitoring neurological activity. Wavelengths particularly near 6.45 microns have been found optimal for cutting all soft tissues. On the other hand, two wavelengths 7.5 and 7.7 microns have been found to cut through bone particularly cleanly.
• The progress in Free electron lasers can help in developing dynamic imaging techniques for diagnosing conditions such as progressive degenerative diseases and cancer.
• Military applications as discussed above.

References

• http://john.mer.utexas.edu/a235fb325d345ac9f0e/ftp/papers/renieri.pdf
• http://www.onr.navy.mil/en/Media-Center/Fact-Sheets/Free-Electron-Laser.aspx
• http://pbpl.physics.ucla.edu/Education/Text/oe_fel.pdf
• http://cas.web.cern.ch/cas/Granada-2012/Lectures/GranadaLectures/WolskiFreeElectronLasers.pdf
• http://en.wikipedia.org/wiki/Free-electron_laser