First lasing of the mid infrared FEL at ELBE was achieved on May 7, 2004. The Radiation Source ELBE at the Forschungszentrum Rossendorf in Dresden is currently under transition from commissioning to regular user operation. Presently the electron linac produces an up to 18 MeV, 1 mA (cw) electron beam which is alotted to generate various kinds of secondary radiation. After the successful commissioning of the bremsstrahlung and channeling-X-ray facilities during 2003 stable lasing has now been observed in the IR range (15 to 22 μm). The oscillator FEL is equipped with two planar undulator units, both consisting of 34 hybrid permanent magnet periods of 27.3 mm (K_rms = 0.3 - 0.8). The distance between the two parts is variable and the gaps can be adjusted and tapered independently. At 19.6 µm an optical power of 3W was out-coupled in a macro pulse of 0.6 ms duration using an electron beam energy of 16.1 MeV and an energy spread of less than 100 keV; the micropulse charge was 50 pC and its width slightly above 1ps. With the installation of a second acceleration module for additional 20 MeV smaller wavelengths will become available in the near future.

The first lasing in the mid IR at the ELBE FEL allows us to specify the parameters of a new undulator for longer wavelengths to complement the U27 undulator which is useful up to about 25 microns. In the longer wavelength region FELs constitute a unique radiation source with appealing properties. Radiation quanta in this range (2 - 10 THz) are appropriate for the low-energy spectroscopy of various interesting modes in solid state quantum structures as well as in complex biological systems. Their study establishes the basis for understanding phenomena in semiconductors and elucidating biological processes of interest for medical innovations. We envisage an electromagnetic undulator with a period of 90 - 100 mm. Using the ELBE beam IR light from 20 to 150 microns and beyond can be produced. To keep the transverse beam extension small the IR beam is to be guided by a partial waveguide inside the undulator. Appropriate bifocal resonator mirrors minimize the mode coupling losses at the exits of the waveguide. Detailed calculations and computer simulations predict an outcoupled laser power of roughly 50 W at 150 microns which will be transported to experimental stations.