Bahrdt, J.; Ivanyushenkov, Y.: Short Period Undulators for Storage Rings and Free Electron Lasers. Journal of Physics : Conference Series 425 (2013), p. 032001/1-6
10.1088/1742-6596/425/3/032001

Abstract:
Short period undulators have the potential to enhance the spectral performance of synchrotron radiation sources significantly. These devices open the range of brilliant hard Xrays at medium energy storage rings, their implementation may reduce the facility length of linac based FELs and they are an essential prerequisite for laser plasma accelerator based FELs. The development of cryogenic permanent magnet undulators has started and the first devices, employing period lengths around 18mm, have been installed in 3rd generation storage rings. Ambitious magnet designs permit even smaller period lengths which improve their performance further. The paper discusses the performance of cryogenic permanent magnet undulators in comparison with superconducting undulators.