Kuhn, C.; Bahrdt, J.; Gaupp, A.; Scheer, M.; Schulz, B.: Development of Advanced Magnet Structures for Cryogenic In-Vacuum Permanent Magnet Undulators. In: Christine Petit-Jean-Genaz ... [Ed.] : Proceedings of IPAC 2014, Dresden, Germany. , 2014. - ISBN 978-3-95450-132-8, p. WEPRO029/2004-2006
http://accelconf.web.cern.ch/AccelConf/IPAC2014/papers/wepro029.pdf

Abstract:
Short period undulators and in particular in-vacuum cryogenic permanent magnet undulators are the upcoming technique for FEL radiators, because they permit a significant reduction of linac and undulator length. For achieving high photon energies with low electron energies (short period lengths, e.g. below lOmm) permanent magnet structures are superior, due to their high surface current density of 16 kA/cm as compared to electromagnetic or even superconducting devices. The geom.etrical to1erances scale with the period length. This requires new fabrication techniques and structure designs, particularly for sub-cm period lengths. Salutions for these demands will be presented and results from a first prototype using various new technologies such as compound poles will be discussed and compared with common approaches.