The 4th generation of light sources will consist of Free Electron Lasers (FELs) using the principle of Self Amplified Spontaneous Emission (SASE). They will be driven by Linacs using low emittance photocathode electron guns. Transverse RMS beam sizes are typically 20-50 µm and the bunch lengths are of the same magnitude. Very long undulators are needed for SASE FELs in order to reach saturation. Their lengths may well exceed 200 m for the X-ray FELs.
At DESY a proof of principle experiment of a SASE-FEL in the VUV regime has been set up at the TESLA Test Facility (TTF). First lasing was observed in February 2000. This device now holds the short wavelength world record for FELs at 80 nm. Its undulator system is described on this page.
