What are Free Electron Lasers (FEL)?

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- GoPhotonics

Jan 16, 2023

A Free electron laser (FEL) is a light source that emits coherent electromagnetic laser pulses with short pulse lengths in the wavelength range from millimeters to x-rays depending on the electron energy. FEL was invented by John Madey in 1971 at Stanford University, and since then many FEL facilities have been constructed for various applications, including biomedical applications. The lasing medium of FEL contains very high-speed electrons moving freely through a magnetic structure. These electrons have a velocity that approaches the velocity of light. FEL has advantages in the terahertz and x-ray wavelength regions, which are not served by conventional lasers.

Structure of a Free Electron Laser 

The structure of FEL consists of a series of magnets that produce a periodic transverse magnetic field called wigglers or undulators which allows the electron beam to pass through. These undulators are kept in between two mirrors that reflect and focus the synchrotron radiation produced by the undulator. One of the mirrors is highly reflecting and the other one partially reflecting. There is an electron accelerator that helps in accelerating the electron beam through the undulators. Figure 1 shows the schematic diagram of a typical FEL structure. 

Figure 1: Structure of a Free Electron Laser

The wavelength of the FEL depends on many factors such as the velocity of the electron beam, the spacing of the wiggler magnets, and the magnetic field. The spacing of the wiggler magnets may usually be a few centimeters, and the total device length is a few meters. 

Working of Free Electron Laser 

An electron beam is introduced by magnets into the laser cavity. These electrons accelerated using an electron accelerator pass through a periodic arrangement of magnets having alternate poles across the beam path that create a side-to-side magnetic field. The direction of the beam is called longitudinal direction and the direction across the beam path is called transverse direction. This magnetic array is called an undulator or wiggler because the electron beam wiggles transversely traveling along a sinusoidal path along the axis of the modulator due to the Lorentz force of the field. Photons are released as a result of the transverse acceleration of the electrons across this path. These photons are monochromatic but incoherent because the electromagnetic waves generated from randomly distributed electrons will interfere constructively and destructively in time. The resulting radiation power has a linear relationship with the number of electrons. The mirrors at both ends of the undulator make an optical cavity that causes radiation to form standing waves. The synchrotron radiation becomes strong enough such that the transverse electric field of the radiation beam interacts with the transverse electron current generated by the sinusoidal wiggling motion. This causes some electrons to gain and others to lose energy to the optical field. The radiation power increases and microbunching happens in the electron bunches. Some of the photons or radiation are extracted through the output mirror.

Energy Level Diagram of Free Electron Laser

Figure 2: Energy level diagram of Free Electron Laser

The electron energy transitions occur in a continuum in free electron laser. Figure 2 above shows the electron energy states in a hydrogen atom. The lower level is level 1 with the lowest energy and is termed the ground state. Level 2, level 3, level 4, etc are levels arranged in the order of increasing energy. The electron energy for a single electron corresponds to 1 electron volt (eV) which is equal to 1.6 x 10-19 Joules which is very tiny. Each electron in each element has its own pattern of energy levels called elemental fingerprints. Electrons can move between levels if they are given or give out the exact amount of energy corresponding to the difference in the energy levels. For a hydrogen atom, if an electron at level 1 is given more than 13.6 eV of energy, the electron will get ionized and forms free electrons. For example, energy jumps A, B, and C are allowed, but D is not possible for this atom. E ionizes the atom with an energy gain of greater than 3.4 eV. The energy of the photon is equal to the energy change between levels. Shorter wavelength photons will have higher energy jumps.

Parameters of Free Electron Laser 

The table below shows the parameters of a free electron laser.

Laser wavelengths2.48 nm to 8 mm
Fractional laser bandwidth10-3 to 10-7
Gain per pass1 - 300 %
Laser gain-medium length1 - 25 m
Pumping methodHigh-energy electron beam
Electron beam peak current0.1 - 800 A
Electron beam energy200 kV to 1 GeV
Electron beam pulse length2 ps to cw
Undulator magnetic period5 mm to 0.2 m
Magnetic field strength0.02 - 1.0 T
Output powerUp to 1 GW (pulsed), up to 10 W (CW)

Features of Free Electron Laser 

  • They can be operated in a very wide wavelength range
  • Large wavelength tuning is possible
  • Outstanding performance in the extreme wavelength region
  • They are extremely powerful and efficient
  • High spectral brightness and coherence

Limitations of Free Electron Laser 

One important limitation of Free Electron Lasers requires a high-quality beam of relativistic electrons, with low angular spreading and very little variation in electron velocity. These sources are very large and expensive. Substantial resources are required to build and operate an FEL. These lasers can be used only at large facilities in the world.

Applications of Free Electron Laser 

Since Free electron lasers produce high-intensity laser pulses, they excel as research tools at submillimeter and far-infrared wavelengths. They are used to conduct different research studies that include measurements of principal excitations in condensed-matter systems, where principal excitations such as plasmons, phonons, magnons, and inter-sub band transitions are possible. Free-electron lasers have been investigated as a potential replacement for synchrotron light sources, which have been the workhorses of protein crystallography and cell biology.

The mid-IR wavelengths of FEL are used for medical and dental treatments using characteristic laser absorption of specific biomolecules and laser surgery of tumors. Using infrared FEL wavelengths around 6.45 micrometers, soft tissues such as skin, cornea, and brain tissue might be cut, or ablated, with little collateral damage to surrounding tissue.

Free electron lasers have applications in the military field also. The US Navy is considering FEL technology as a suitable directed energy weapon for antiaircraft and antimissile defense. Experiments such as the formation of multiple core holes in atoms, multiphoton ionization of K-shell electrons, and giant Coulomb explosions in atomic clusters are all possible in the field of atomic physics using free electron lasers.