Pushing the Limits of Short Period Permanent Magnet Undulators

Short period undulators to be used as FEL radiators permit lower electron energies and, thus, reduced linac and undulator lengths. The first X-ray FEL facility based on in-vacuum permanent magnet undulators has gone into operation, already (SACLA). Another in-vacuum undulator based X-FEL is planned (SWISS-FEL). The in-vacuum undulators have period lengths of 18mm (SACLA) and 15mm (SWISS-FEL), respectively. In the future the period length will be pushed further into the sub-cm regime. The technical challenges of such devices will be discussed: New magnet materials such as PrFeBmagnets are employed. They show their superior characteristics at cryogenic temperatures. Sophisticated pole geometries are under discussion. Geometric and magnetic tolerances become tighter and the construction and shimming concepts have to be revised. A redesign of magnet measurement systems is needed as well. Recently, a 9mm period length 20 period cryogenic undulator has been built in a collaboration of LudwigMaximilian-University Munich and Helmholtz-Zentrum Berlin. The potential and the challenges of sub-cm permanent magnet undulators will be illustrated based on first results from this prototype. INTRODUCTION The synchrotron radiation of 3 generation light sources is produced predominantly from permanent magnet undulators and this is the case also for all operational or planned soft and hard X-ray free electron lasers. Planar permanent magnet undulators are used for more than 30 years. Variably polarizing devices of the APPLE II type have become a standard source at many light sources [1]. Quasi-periodic structures have been developed for the suppression of higher harmonics at the sample and several quasi-periodic APPLE structures are in operation [2-4]. Planar in-vacuum undulators have been developed at SPRING-8 and are used in many synchrotron radiation facilities. Alternatively, electromagnetic or superconducting devices can be used. A superconducting undulator (SCU) is in operation at ANKA since 2005 [5] and further SCUs are under construction at ANKA [6] and the APS [7]. Undulators based on Nb3Sn have been studied and high temperature superconductor tapes for the application in small period undulators are considered [8]. A few electromagnetic undulators or electromagnet permanent magnet hybrids are used in the low photon energy (large period length) range. In most cases, however, permanent magnet undulators are the only choice and this will be the case for the next years. Permanent magnets can be modelled as blocks carrying surface currents of at least 10kA/cm. Electromagnets cannot compete due to cooling problems, when the dimensions become small. SC undulators are promising but the technology is still under development. Currently, the development in permanent magnet undulator technology concentrates on planar cryogenic permanent magnet undulators based on NdFeB or PrFeB with period lengths of about 16mm and on small period small gap devices with period length below 10mm. Another goal of development is the field enhancement of variably polarizing devices such as APPLE III [9] (out of vacuum) or DELTA [10] (in-vacuum) undulators. Storage ring undulators have to provide enough transverse space for injection and a large horizontal good field region in order to enable high injection efficiencies. The magnetic gap must be large enough not to interfere with storage ring operation and to avoid radiation damages. The requirements for single pass devices are different and the undulator design has to be adopted appropriately. A large transverse aperture or a good field region of typical 1-2cm is not needed and the magnetic gap is only limited by wakefield effects. In this article we concentrate on the development issues of planar small period small gap devices without addressing specific applications though keeping in mind 2 harmonic afterburners and FEL radiators for laser