Free electron laser

Functional principle of the free-electron laser. The electron beam is generated in a particle accelerator and passes through the planar undulator on a periodic path (red). The X-ray beam (orange) is generated by the transverse movement.

A free-electron laser (engl .: free-electron laser , short FEL) is a radiation source, the synchrotron radiation with very high brilliance generated. Since free electrons have no fixed energy levels, the emitted radiation can be continuously tuned. Currently (2017) wavelengths below 1 Angstrom are possible.

The FEL is called a laser because of the coherence of this radiation and the dependence of the gain on the number of photons present . In contrast to conventional lasers, however, it does not have a laser-active medium in which there is population inversion . Therefore, there is no stimulated emission either .

Central components of an FEL are the electron source , usually a high-energy particle accelerator , and an interaction area in which part of the kinetic energy of the electrons is converted into photons. This usually happens through an alternating magnetic field ( undulator ), which forces the electrons into a transverse movement, in which synchrotron radiation is emitted. However, the generation of photons can also be induced by a waveguide coated with a dielectric ( Cherenkov FEL ). In the broadest sense, the first coherent radiation sources, traveling wave tubes and magnetrons , can also be understood as free-electron lasers. The function of an FEL that uses an undulator to generate radiation is explained below.

The FEL was invented by John Madey at Stanford University in the early 1970s and a prototype was built.

Structure and functionality

The schematic structure is shown in the sketch above. A packet of electrons is accelerated to relativistic speed in one or more accelerators and then passed into an undulator . The sinusoidal movement of the electrons and the associated circular acceleration emit high-energy photons, which are known as synchrotron radiation .

For a high brilliance of the emitted radiation, the electrons in the particle package must have the lowest possible energy spread, a low emittance and a high peak current. Generating such an electron bunch is complex, because due to the mutual Coulomb repulsion of the electrons, a particle bunch with the required peak current can not be generated directly in the particle source of the accelerator. Instead, a packet of electrons is first generated with a small current, which is immediately accelerated to ultra-relativistic energies and then compressed longitudinally. This compression shortens the electron bundle and increases the peak current to the same extent, which is possible due to the Coulomb repulsion , which is now greatly reduced due to relativistic effects . If necessary, this sequence of acceleration and compression is repeated several times (up to three times for X-ray FELs), which makes free-electron lasers a complex and expensive system. At FLASH z. For example, before the first compression, the electron packet is first accelerated to 145 MeV, then further accelerated to approx. 450 MeV and then compressed again. Finally, the now fully compressed beam is accelerated to the final energy (maximum around 1.2 GeV).

In the undulator, the electron beam is alternately arranged magnets in a periodic transverse movement offset ( engl. To undulate ), wherein the electron emitting synchrotron radiation. The wavelength of the emitted light is given by ${\displaystyle \lambda _{r}}$

${\displaystyle \lambda _{r}={\frac {\lambda _{u}}{2\gamma ^{2}}}\left(1+{\frac {K^{2}}{2}}\right)}$,

with the period of the undulator, the Lorentz factor and the so-called dimensionless undulator parameter . This is given by ${\displaystyle \lambda _{u}}$ ${\displaystyle \gamma }$ ${\displaystyle K}$

${\displaystyle K={\frac {eB_{0}\lambda _{u}}{2\pi m_{e}c}}\approx 0.93B_{0}[\mathrm {T} ]\lambda _{u}[\mathrm {cm} ]}$,

with the magnetic field of the undulator . The factor arises from the fact that , on the one hand, the electron sees an undulator contracted by Lorentz , and on the other hand, the emitted light in the laboratory system is shifted by Doppler . Due to the relativistic movement of the electrons, the emitted radiation is almost completely directed forward along the electron path. ${\displaystyle B_{0}}$ ${\displaystyle \gamma ^{-2}}$ ${\displaystyle \gamma ^{-1}}$ ${\displaystyle \gamma ^{-1}}$

In the FEL, the undulator is built very long, so that there is an interaction between the emitted radiation and the electron packet. The electron packet is microstructured through interaction with the generated radiation, that is, it is divided into thin slices that are oriented perpendicular to the direction of flight. The distance between these disks is equal to the wavelength, so that all electrons in the packet can coherently emit at the same time . Due to the in-phase emission of the radiation, the amplitudes of the individually generated waves add up and not the intensities, as would be the case with random radiation that is not emitted in the correct phase. The result is that the intensity of the emitted radiation in the FEL increases proportionally to the square of the number of emitting electrons and no longer linearly. This creates coherent radiation of high brilliance.

The wavelength of an FEL can be tuned by varying the energy of the electrons or the magnetic field of the undulator, whereby the tuning range is in principle not limited. However, technical factors such as the available electron energies and the tuning range of the undulator magnetic field limit the tuning range.

distribution

In 2006 there were 21 free-electron lasers worldwide, and 15 more systems were under construction or in the planning stage. In principle, free-electron lasers cover large parts of the spectral range , but are optimized for a specific range. The Particle Physics Lab FEL in Dubna works in the millimeter range, the FLASH ( free-electron laser in Hamburg ) at DESY in the UV range (4.12 to 30 nm). The currently short-wave radiation is emitted at the European XFEL (X-ray FEL), whose injector is also located at DESY in Hamburg. It reaches a wavelength of 0.05 nm. The X-ray flashes from the European XFEL are so small that even atomic details can be seen.

Military use

FEL technology has been evaluated as an anti-aircraft candidate by the US Navy . Significant progress has been made in increasing the power (the FEL of the Thomas Jefferson National Accelerator Facility was able to demonstrate over 14 kW power) and it now also appears possible to build compact multi-megawatt FEL weapons. On June 9, 2009, the Office of Naval Research announced that it had signed a contract with Raytheon to build an experimental 100 kW FEL. On March 18, 2010, Boeing Directed Energy Systems announced the completion of a special design for maritime use. The presentation of a complete prototype is scheduled for 2018.

Web links

• Free electron laser determination_Strahlparameter_am_FEL (PDF file; 1.2 MB)
• Development of a table-top FEL with laser-accelerated electrons at the LMU / MPQ (English)
• XFEL - Introduction of X-ray laser light through self-amplification
• Free-electron laser from the ELBE radiation source in the Helmholtz Center Dresden-Rossendorf
• The World Wide Web Virtual Library: Free Electron Laser research and applications (English)
• FLASH at DESY
• European XFEL website
• Facts and figures about the European XFEL
• Linac Coherent Light Source website

Individual evidence

1. ^ Free Electron Lasers 2002 , Proceedings of the 24th International Free Electron Laser Conference, and the 9th FEL Users Workshop, Argonne, Illinois, USA, September 9-13, 2002.
2. ↑ John Madey, Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field , J. Appl. Phys. 42, 1971.
3. ↑ Z. Huang, K.-J. Kim: Review of x-ray free-electron laser theory . In: Physical Review Special Topics - Accelerators and Beams . 10, 2007, p. 034801. doi : 10.1103 / PhysRevSTAB.10.034801 .