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Cryogenic Permanent Magnet Undulator Development at HZB/BESSY II

The costs of a synchrotron radiation facility scales approximately linearly with the length (FEL) or the circumference (storage ring) of the machine. It is always beneficial for the reduction in overall expenses to utilize short period in-vacuum undulators (IVUs) for X-ray production. This is the reason for the success of the IVU development which was started almost 20 years ago in Japan [1–3]. Today, IVUs are implemented into nearly all third-generation storage rings. Ten years ago, the concept of cryogenically cooled permanent magnet undulators (CPMUs) was proposed [4]. The magnetic properties of rare earth magnets (i.e., the remanence and the coercivity) improve substantially at low temperatures. The remanence increases by about 15%, whereas the coercivity grows by a factor of three to four. Due to the performance gain and the low technical risk of CPMUs, such devices are under development all over the world. The first generation of CPMUs, with period lengths well below 20 mm, is successfully operated at ESRF [5, 6], PSI [7], DIAMOND [8], SOLEIL [9], and SPring-8 [10].