Bahrdt, J.: Pushing the Limits of Short Period Permanent Magnet Undulators. In: 33rd International Free Electron Laser Conference FEL 2011, Shanghai, China. August 22 to August 26, 2011. , 2011, p. [1-7]

Short period undulators to be used as FEL radiators permit lower electron energies and, thus, reduced linac and undulator lengths. The first X-ray FEL facility based on in-vacuum permanent magnet undulators has gone into operation, already (SACLA). Another in-vacuum undulator based X-FEL is planned (SWISS-FEL). The in-vacuum undulators have period lengths of 18mm (SACLA) and 15mm (SWISS-FEL), respectively. In the future the period length will be pushed further into the sub-cm regime. The technical challenges of such devices will be discussed: New magnet materials such as PrFeB-magnets are employed. They show their superior characteristics at cryogenic temperatures. Sophisticated pole geometries are under discussion. Geometric and magnetic tolerances become tighter and the construction and shimming concepts have to be revised. A redesign of magnet measurement systems is needed as well. Recently, a 9mm period length 20 period cryogenic undulator has been built in a collaboration of Ludwig- Maximilian-University Munich and Helmholtz-Zentrum Berlin. The potential and the challenges of sub-cm permanent magnet undulators will be illustrated based on first results from this prototype.