Fritz-Haber-Institut der Max-Planck-Gesellschaft

FEL

The FHI free-electron laser (FEL) facility

Overview


Photograph © M. Wesemann
The free-electron laser (FEL) at the Fritz Haber Institute (FHI) generates intense pulses of infrared (IR) radiation of widely tunable wavelength (from 3 µm to 60 µm, as of September 2017). Unlike conventional lasers, where the radiation is produced in a gas, liquid, or solid, in an FEL it is generated by an electron beam propagating freely through a vacuum tube. In a device called an undulator, strong magnetic fields of alternating polarity force the electrons to undergo a wiggling (undulating) motion, thereby causing the emission of radiation. The radiation wavelength can be tuned simply by varying the electron energy or the magnetic field strength. Before entering the undulator, however, the electrons must be accelerated to almost the speed of light, requiring a complex electron accelerator. Since 2013 such an installation has been operational at the Fritz Haber Institute.

The generation of molecular vibrational spectra is one of the main uses of the FHI FEL. The IR spectral region is often referred to as the "molecular fingerprint" region, as it is the region in which the fundamental vibrational modes of molecules, clusters or solid materials are located. These vibrational modes are directly connected to the forces that hold the atoms together and to their geometrical structure.

The implementation

Ever since the first demonstration of a free-electron laser in 1976 at Stanford University, it has been realized that this type of laser can potentially deliver high-power tuneable radiation over a large spectral range. A layout of the FHI FEL is shown in the figure:

Electrons are emitted from the electron gun. After acceleration to an energy of up to 50 MeV (mega-electronvolt) by two linear accelerators (LINACs), the electron beam is bent and injected into the resonator, consisting of two high-reflectivity mirrors at each end of the undulator. The magnetic field in the undulator is perpendicular to the direction of the electron beam and periodically changes polarity a large number of times along its length. This causes a periodic deflection, a `wiggling' motion, of the electrons while traversing the undulator. This wiggling causes the emission of light and its wavelength is determined by the electron energy as well as by the undulator parameters. The initial weak radiation is captured in the resonator and amplified by interaction with successive electron bunches. A small hole in one of the two resonator mirrors couples out some light, which is sent to user experiments.

The following research groups and companies have contributed:

• Advanced Energy Systems: responsible for the electron beamline,
• STI Optronics: design and supply of the undulator,
• Bestec GmbH: design and construction of the resonator optics.
• FELBE at the Forschungszentrum Rossendorf: technical consulting,
• DGI Bauwerk: the architects for the building,