A STUDY ON FIELD ERROR OF BULK HTSC STAGGERED ARRAY UNDULATOR ORIGINATED FROM VARIATION OF CRITICAL CURRENT DENSITY OF BULK HTSCS* T. Kii # , R. Kinjo, M. A. Bakr, Y.W. Choi, K. Yoshida, S. Ueda, M. Takasaki, K. Ishida, N. Kimura, T. Sonobe, K. Masuda, H. Ohgaki, Institute of Advanced Energy, Kyoto, Japan. Abstract The bulk high-temperature superconductor staggered array undulator (bulk HTSC SAU) has potential to generate strong periodic magnetic field in short period and to control K value without mechanical gap control structure [1]. However, availability of the bulk HTSC magnets having matched performance of critical current density is a problem to be solved. In this study, we have numerically estimated influence of variation of critical density upon field error using a loop current model based on the Bean model for a type-II superconductor. It was numerically found that the field error was naturally suppressed. INTRODUCTION In order to realize strong periodic magnetic field in short period, we proposed the bulk high-temperature superconductor staggered array undulator (bulk HTSC SAU) [2, 3]. The bulk HTSC SAU consists of stacked bulk superconductor magnets inserted in a solenoid magnet as shown in fig.1. Magnetization vector x Undulator field y z External solenoid External solenoid Bulk magnet Figure 1: Schematic drawing for principle of the bulk HTSC SAU. Stacked bulk HTSCs are magnetized by an external solenoid. The transverse magnetic field (B y ) is produced by these bulk SC magnets. The bulk HTSC SAU works as follows. The stacked array is cooled down below the critical temperature in the presence of magnetic field B start . Next, the solenoid field strength is changed to B end . Then, superconducting loop current appears from outer rim toward to inner part of each bulk HTSCs until the supercurrent compensates for the effect by the field change from B start to B end . As the results, the transverse field component B y is generated by the superconducting current loops excited in the each bulk HTSC magnet. Here, it is noted that the superconducting currents is determined by the field difference ΔB = |B start - B end | which is defined by the solenoid and the magnetic field generated by stacked bulk HTSCs. The features of the undulator are listed below. • Undulator field can be generated using single solenoid magnet. • No mechanical structure is required in controlling undulator field. In order to prove the principle of the bulk HTSC staggered array undulator and estimate the performance, a prototype undulator consisting of a 11 periods of stacked array, a liquid nitrogen cooled vacuum duct, and a normal conducting solenoid was developed [2]. The schematic drawing of the prototype undulator is shown in fig. 2. Detail of copper pieces and DyBaCuO bulk superconductors is shown in fig. 3. Typical trapped field distribution of bulk HTSCs at 77K is shown in fig 4. The average and standard deviation of maximum trapped field of 22 pieces of bulk superconductor were 0.11 T and 0.017 T respectively. Pt100 temperature sensor Linear motion drive Li N2 inlet z y Normal conducting soleoid Hall generator Stacked array of HTSC and copper spacer Thermal insulator Li N2 outlet Figure 2: Schematic view of the prototype undulator. Bulk HTSC Copper insulator 25.2 mm 2.5 mm 4.0 mm 14.0 mm Figure 3: Half period of the stacked array. ___________________________________________ *Work supported by Grant-in-Aid for Scientific Research (B) and for JSPS Fellows # kii@iae.kyoto-u.ac.jp THPC02 Proceedings of FEL2010, Malmö, Sweden 648 FEL technology II : Undulator and Beamline