IMPROVED HOMOGENEITY OF PERMANENT MAGNETS FOR UNDULATORS AND WIGGLERS

The homogeneity of permanent magnets for use in undulators and wigglers was significantly improved in close collaboration between industry and scientific institutes throughout the last three years. Magnets with a variation of remanence of less than +/1%, angular errors of +/1° and hot-/cold-side effect of +/1% can be produced now. A variation of the magnetic angle of less than +/0.5 ° is possible for some products. The development was assisted by improved characterization equipment for magnetic dipole moments and magnetic inhomogeneities. MOTIVATION Beam guiding applications using permanent magnets for wigglers or undulators have very high demands on the homogeneity, angular errors and hot-/cold side effects of the material. Any deviation from ideal properties will cause extra deflections of the electron beam leading to trajectory and/or phase errors and require extra treatment such as shimming or pole tuning. So far, for commercially available permanent magnets these properties as well as the accuracy of the geometrical dimensions were limited. Therefore a big additional effort was needed to reach final specifications: Magnets were characterized, sorted according to their measured errors and finally sophisticated shimming and/or tuning procedures were applied to obtain the optimum performance of the wiggler or undulator. More perfect permanent magnet material would have a big impact and eventually simplify the whole manufacturing process of insertion devices considerably. There is another aspect: Large projects such as the European XFEL currently under construction in Hamburg [1] require very large quantities of high quality material with very homogeneous properties. These quantities are well beyond standard production batch sizes. For example for the SASE2 system for the XFEL requires about 10 tons of such material. This is well above the present production batch size of ≈ 2 tons. In order to improve permanent magnet properties a research project was launched and funded by the German ministry of education and research (BMBF). It was a joint project between Vacuumschmelze GmbH & Co KG, Hanau, DESY, Hamburg and BESSY, Berlin. The most relevant results for applications in wigglers and undulators applications are presented in this paper. STEPS TOWARDS MORE HOMOGENEOUS MAGNETS The aspects of homogeneity of magnets may be divided into mechanical properties (dimensions, volume, rectangularity), global/integral magnetic properties (remanence, magnetic moment, angular deviation) and local magnetic properties (hot-/cold-side effect). These properties were investigated and improved in several work packages. Table 1 gives an overview over the state of the art in 2004, the objectives of the project and the actually achieved values. Table 1: improvement of characteristics Status 2004 targets Final status 2007 *) Variation of absolute Remanence: from batch to batch ± 3 % ± 0.5 % n.a. for 10 t batches inside a batch ± 2 % ± 0.5 % ± 0.4 % Angular misalignment of magnetization ± 1.5 ° ± 0.2 ° ± 0.5 ° Hot-/Cold-side effect ± 5 % ± 0.5 % ± 1 % Mechanical tolerances ± 50 μm ± 10 μm ± 25 μm *) achieved in single lots, not simultaneous in all cases Powder and Sinter Technology The remanence of permanent magnets is dominantly influenced by its chemical composition, its sinter density and the degree of orientation of the individual powder grains. Given a close control of the chemical composition, the full material density may be controlled by an optimized sinter process. The orientation of grains is strongly influenced by the pressing procedure (see below). Thus, inside a single batch, the remanence may be controlled to a narrow variance. If the amount of magnet material exceeds several tons, for example as requested for the XFEL project, a powder-blending method was developed. It allows to adjust a stable remanence level by combining a set of several powder batches (Fig. 1). + This project was funded by the German ministry of education and research (BMBF) WEPC101 Proceedings of EPAC08, Genoa, Italy 02 Synchrotron Light Sources and FELs 2234 T15 Undulators and Wigglers Figure 1: Powder blending method for large lots.