Challenge of In-vacuum and Cryogenic Undulator Technologies

An in-vacuum undulator (IVU) opens the utilization of high-brilliance X-rays in the medium energy storage rings. The development of a short-period undulator with low phase error becomes important to bring X-ray into a new unprecedented brilliant light source in an ultimate storage ring (USR). NdFeB or PrFeB cryogenic permanent magnet undulators (CPMUs) with a short period have been developed worldwide to obtain high brilliance of undulator radiation. A CPMU has high resistance against beam-induced heat load and allow to operate at a narrow gap. In a low emittance or ultimate storage ring, not only the performance of an undulator but the choice of the lattice functions is very important to obtain high bril-liance of synchrotron radiation. The optimum betatron functions and zero dispersion function shall be given for a straight section at IVU/CPMUs. In this paper, the relevant factors and design issues for IVU/CPMU will be discussed. Many technological challenges of a short-period undulator associated with beam induced-heat load, phase errors, and the deformation of in-vacuum girders will also be presented herein.