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Free electron laser

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

A free electron laser, or FEL, generates tunable, coherent, high power radiation, currently ranging in wavelength from millimeters to the 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.

Contents

FEL 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 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.

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.)

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.

From the klystron to the free electron laser

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 in. If energy is converted to the EM-wave, this device is called a klystron, otherwise it is an linear electron accelerator (linac).

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.

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.

Energy flow at the XFEL at
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.

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. [link]

Patents

Further reading

See also

References

 

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