Overview

Principle of Free Electron Lasers

A Free Electron Laser (FEL) generates tunable, coherent, high power radiation, currently spanning wavelengths from millimeter to some 80 nanometers. It can have the optical properties characteristic of conventional lasers such as high spatial coherence and a near diffraction limited radiation beam. It differs from conventional lasers in using a relativistic electron beam as its lasing medium, as opposed to bound atomic or molecular states, hence the term free-electron.

The electrons travel through a periodically alternating magnetic field of an undulator or wiggler as shown in fig. 1. In this field the electrons are accelerated transversally so that they produce synchrotron radiation called spontaneous emission. The radiation is stored in an optical cavity which allows the coupling to the electrons and amplification of the electromagnatic wave. The optical cavity plays the same role as in conventional lasers. The wavelength of the radiation is primarily determined by the magnetic field strength of the undulator and the energy of the electrons.

Despite the fact that existing OPOs and OPAs based on convetional laser systems can already cover a wide range of wavelengths as well as repetition rates and provide even shorter pulses than Free Electron Lasers, FELs still remain important as far as applications with the need of highest repetition rates up to GHz or highest average power - more than 1.7 kWs have been demonstrated in the infrared - are concerned. Furthermore the use of techniques like high gain harmonic generation (HGHG) and SASE will be capable of producing radiation at wavelength as short as a few nanometers.

IR-FEL at the S-DALINAC

The FEL at the Superconducting Darmstadt Linear Accelerator S-DALINAC is set up in a bypass system to the first recirculating beamline. Two dipole magnets direct the electron beam onto the optical axis and into the undulator. The third dipole deflects the electrons into a beam dump.

The major properties of the electron beam, the undulator and the optical cavity are listed in the following table: