Free electron laser

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X-ray free electronic laser schema of operation
X-ray free electronic laser schema of operation

A free electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as 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, which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free electron. This gives them the widest frequency range of any laser type, and makes many of them widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to ultraviolet, to soft X-rays.

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[edit] Beam creation

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 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 (bremsstrahlung or synchrotron radiation, but not in the most common sense of either term).

Viewed relativistically in the rest frame of the electron, the magnetic field can be treated as if it were a virtual photon. The collision of the electron with this virtual photon creates an actual photon (Compton scattering). 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. Because compton scattering is complicated in itself, it is easier to say that the electrons are forced onto a sinus path by the undulator and then switch in a rest frame moving along the undulator, where the electrons are oscillating, but not moving otherwise, and emit dipole radiation, and than switch back into the rest frame of the undulator to see that this dipole radiation is transformed into a forward emitted radiation of shorter wavelength.

Since the photons emitted are related to the electron beam and magnetic field strength, an FEL can be tuned, i.e. the frequency or color can be controlled.

What makes it a laser (light amplification by stimulated emission of radiation) is that the electron motion is in phase (coherent) with the field of the light already emitted, so that the fields add coherently. Since the intensity of light depends on the square of the field, this increases the light output. (Surprisingly, quantum mechanics is not required in this explanation.). In the rest frame moving along the undulator any radiation will still move with the speed of light and pass over the electrons and lets them communicate to get in synchronization. Often same light (that is radiation) is introduced from the outside. Depending on the position along the undulator the oscillation of an electrons is in phase or not in phase with this radiation. The light either tries to accelerate or decelerate these electrons. It thereby gains or loses kinetic energy, so it moves faster or slower along the undulator. This causes the electrons to form bunches. Now they are synchronized, and will in turn emit synchronized (that is coherent) radiation.

[edit] 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.

[edit] From the klystron to the free electron laser

[edit] Basics

In a klystron an electron beam is accelerated by a 200 kV DC electric field. An electromagnetic wave interacts with it, modulating its velocity. In a drift tube this velocity distribution is converted to a density modulation. In a second interaction region energy can be converted from the electron beam to the EM-wave or vice versa depending on the relative phase with which both are fed. If energy is converted to the EM-wave, this device is called a klystron, otherwise it is an linear electron accelerator (linac).

[edit] Interaction devices

In a klystron or linac the wavelength of the EM-wavelength is larger than the electron beam and various waveguide structures can be used to slow down the EM-field to speed of the electron density (group) velocity and at the same time provide E-fields in the direction of the electron motion.

In a gyrotron or free electron laser the EM-wavelength is smaller than the electron beam and the electrons have to be manipulated. Magnetic fields force them on a sinusoidal path, so as the EM-wave overtakes them and the E-vector changes sign, the electrons change direction.

Most interaction devices are tunable, but only a family of waveguides called traveling wave tubes allows one octave wide instant bandwidth and thus short pulses, but have cooling problems as they consist of helical wires or wire chambers.

[edit] Quantum noise

The amplified wave can be fed back thus producing an oscillator. Free electron lasers in the visible region and above are so energy hungry that operation is only possible for short durations. Lasers start up from quantum noise (optical shot noise), which is damped over time, which these energy hungry beasts don’t have, producing very unstable output.

[edit] 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. When the field extracts enough energy from the electrons over a single pass such that the field amplitude cannot be regarded as constant during the FEL process, the FEL is said to operate in the high-gain regime. In this case one can couple the single particle equations of motion to Maxwell's equations by describing the beam phase space via the Klimontovich distribution and using this distribution to source the paraxial wave equation for the slowly-varying electric field amplitude. Thereby the paraxial wave equation, together with the continuity equation, completely determine the dynamics of the field and beam during the FEL process.

[edit] Self-amplified spontaneous emission

It is a fascinating fact that even if the initial field amplitude is zero, the FEL can still generate a laser through the process of Self-Amplified Spontaneous Emission (SASE); whereby, the (classical) shot noise due to density perturbations in the electron beam causes a noisy signal to be initially radiated. The FEL process preferentially selects one, transversely coherent mode from this noise to amplify till the energy spread in the beam, created by the FEL interaction, dominates, causing the FEL to saturate at some high power. The Linac Coherent Light Source (LCLS), currently being built at the Stanford Linear Accelerator, will operate as a SASE FEL, radiating at wavelengths down to 1.5 angstroms.

[edit] Seeded FELs

One problem with SASE FELs is the lack of temporal coherence due to the 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. Although, this method becomes a problem at x-ray wavelengths because of the lack of conventional x-ray lasers. Of course this problem can also be overcome by employing the method of High Gain Harmonic Generation (HGHG), whereby one first uses an undulator to create bunching at higher harmonics of the seed laser, then in an upstream undulator tuned to a higher harmonic of the seed one uses this bunching to radiate at that higher harmonic. For example starting with a 240 nm UV seed, one could "go to the eighth harmonic" and radiate in the second undulator at 30 nm XUV light. However from available UV seed(around 200 nm), one needs over one thousand harmonic number to reach X-ray wavelength range. Multiple stages HGHG scheme can be used repeatedly to achieve high order harmonic number(Cascade HGHG). Output radiation of one stage becomes the seed laser of the next stage. The total harmonic equals the product of harmonic numbers of each stage.

[edit] Energy flow at the XFEL at DESY

The big picture (ed. Numbers may be incorrect due to author speculation)

Lets start with a 10 kV 3 phase 50 Hz outlet. Solid state technology converts it to 200 kV 1 kHz 1 µs square pulse voltage. This EM-energy is converted to kinetic electron energy. Klystrons convert this to 2 GHz AC EM-waves. A Linac converts this EM-wave energy to a high energy electron beam energy. A free electron laser converts this energy to 100+ THz EM-Waves.

[edit] Medical applications

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. It was reported that at infrared wavelengths, water in tissue was heated by the laser, but at 915, 1210 and 1720 nm, subsurface lipids were differentially heated more strongly than water. The possible applications include the selective destruction of sebum lipids to treat acne, as well as targeting other lipids for the treatment of cellulite and atherosclerosis. [1]

[edit] Military applications

FEL is also considered by US Navy as a good candidate for anti-missile directed-energy weapon. Significant progress is being made in increasing FEL power levels (already at 10 kW) and it should be possible to build compact multi-megawatt class FEL lasers (Airborne megawatt class free-electron laser for defense and security).

[edit] Patents

[edit] See also

[edit] Further reading

  • Boscolo, et al., "Free-Electron Lasers and Masers on Curved Paths". Appl. Phys., (Germany), vol. 19, No. 1, pp. 46-51, May 1979.
  • Deacon et al., "First Operation of a Free-Electron Laser". Phys. Rev. Lett., vol. 38, No. 16, Apr. 1977, pp. 892-894.
  • Elias, et al., "Observation of Stimulated Emission of Radiation by Realistic Electrons in a Spatially Periodic Transverse Magnetic Field", Phys. Rev. Lett., 36 (13), 1976, p. 717.
  • Gover, "Operation Regimes of Cerenkov-Smith-Purcell Free Electron Lasers and T. W. Amplifiers". Optics Communications, vol. 26, No. 3, Sep. 1978, pp. 375-379.
  • Gover, "Collective and Single Electron Interactions of Electron Beams with Electromagnetic Waves and Free Electrons Lasers". App. Phys. 16 (1978), p. 121.
  • "A Unified Theory of Magnetic Bremsstrahlung, Electrostatic Bremsstrahlung, Compton-Raman Scatering and Cerenkov-Smith-Purcell Free Electron Laser".
  • "The FEL Program at Jefferson Lab" [2]

[edit] References

[edit] External links

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