Different focusing solutions for the TTF-FEL undulator

While aiming at shorter wavelengths, the SASE-FEL undulators become longer. Therefore, maintaining the small electron beam sizes required, the question of what kind of focusing structures should be used for optimum performance becomes of increasing interest. Present SASE FEL undulators that have been developed and proposed for future FELS both include integrated focusing (TTF-FEL), separate function quadrupoles combined with the natural undulator focusing (LEUTL) and triplet or FODO structures. In this contribution, the consequence of the different undulator designs in terms of alignment (tolerances) and flexibility will be discussed