Insertion devices for 4/sup th/ generation light sources

The next generation of light sources will consist of Free Electron Lasers (FEL's) using the principle of Self Amplified Spontaneous Emission (SASE). They will be driven by Linacs using low emittance photocathodes. Transverse RMS beam sizes are typically 20-50 /spl mu/m and the bunch lengths are of the same magnitude. Worldwide several projects are under construction or in a design phase to cover the whole spectral range down to 0.1 nm. Very long undulators are needed for SASE FELs in order to reach saturation. Their lengths may easily reach 100 m for the X-ray FELs. In order to minimize the total length and maximize output power they must meet two criteria: First, tough magnetic specifications must be fulfilled in order to have optimum overlap between electron and laser beam. Second, additional external strong focusing is required in order to keep the electron beam size small over the whole undulator length. In this contribution problems related with the design of very long undulators are addressed. Alternative ways of providing the strong focusing are outlined and special attention is given to the magnetic design of undulators with integrated strong focusing. As an example the work done at the undulator under construction for the FEL at the TESLA Test Facility in Hamburg is presented.