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]. Short period undulators are of particular interest for next generation compact electron sources such as laser plasma accelerators [11, 12]. A few years ago, HZB, Ludwig Maximilian Universität München LMUM , and Max Planck Institut für Quantenoptik MPQ launched an R amp;D project on short period CPMUs to exploit the potential of these devices as radiators in a free electron laser FEL . The activities started with a material development with the magnet manufacturer Vacuumschmelze. Two fixed gap prototypes were built and tested. In the second prototype, a new assembling technique for complicated shortperiod pole structures was validated. Currently, two full scale CPMUs are under construction at HZB. Innovative magnet field measurement equipment for field characterization under UHV conditions at low temperatures is required. Preliminary tests of a new, 2 m long in vacuum Hall probe bench have been conducted. An in vacuum stretched wire system is currently under construction. This article will focus on the CPMU related developments at HZ