Enhancing the Radiation Resistance of Undulator Permanent Magnets by Tilting the Easy Axis of Magnetization.

The undulator is a magnetic device usually consisting of a series of rare-earth permanent magnets (REPMs) arranged to generate a sinusoidal magnetic field, and is installed in synchrotron radiation and free electron laser facilities to periodically deflect high-energy electrons. Because the undulator is operated under a high-radiation environment, it is important to take possible measures to avoid the quality degradation, in particular the radiation damage of REPMs. We present a simple scheme to enhance the radiation resistance of REPMs in undulators, in which the easy axis of each REPM is tilted by 45°.

Radiation-induced magnetization reversal causing a large flux loss in undulator permanent magnets.

We report an unexpectedly large flux loss observed in permanent magnets in one of the undulators operated in SACLA, the x-ray free electron laser facility in Japan. Characterizations of individual magnets extracted from the relevant undulator have revealed that the flux loss was caused by a homogeneous magnetization reversal extending over a wide area, but not by demagnetization of individual magnets damaged by radiation. We show that the estimated flux-loss rate is much higher than what is reported in previous papers, and its distribution is much more localized to the upstream side.

Analyses of the factors for the demagnetization of permanent magnets caused by high-energy electron irradiation.

Demagnetization owing to high-energy electron irradiation has been analyzed for permanent magnets used in insertion devices of synchrotron radiation sources, using the Monte Carlo code FLUKA. The experimental data of a thermally treated Nd(2)Fe(14)B permanent magnet with a copper or a tantalum block at electron energies ranging from 2 to 8 GeV were compared with the calculation data of the absorbed doses, photoneutron production distributions and star densities. The results indicate that low-energy photoneutrons and bremsstrahlung photons are not involved in the demagnetization process, and suggest that the star density owing to the photoneutrons is strongly correlated with the demagnetization process.