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TY - UNPB AU - Kinjo, R. AU - Asano, Y. AU - Bizen, T. AU - Hara, T. AU - Hasegawa, T. AU - Itoga, T. AU - Kagamihata, A. AU - Kida, Y. AU - Seike, T. AU - Tanaka, T. AU - Watanabe, T. ED - Bishofberger, Kip:Carlsten, Bruce (LANL, Los Alamos, NM, USA)[ORCID:0000-0001-5619-907X] ED - Schaa, Volker RW TI - Radiation-Induced Magnetization Reversal Causing a Large Flux Loss in Undulator Permanent Magnets J2 - Proc. of FEL2017, Santa Fe, NM, USA, August 20-25, 2017 C1 - Santa Fe, NM, USA T2 - International Free Electron Laser Conference T3 - 38 LA - english AB - A large degradation of undulator field up to 35 percent was observed at the entrance of the in-vacuum undulators in SACLA; although this may be because of the radiation-induced demagnetization, it is much larger than what is expected from the former irradiation experiments. From the measurement of the surface field profile of individual permanent magnets (PMs) and the numerical calculations, we found that not the demagnetization but the magnetization reversal in small areas of the magnets is the reason of the large degradation of the undulator field. Numerical and experimental studies suggest that the magnetization reversal is a highly nonlinear process with respect to the reverse field in the PM, which is applied to the PM in the hybrid array. The experimental study also shows that a 10-cm stainless steel block placed just in front of the PMs significantly impedes the progress of the magnetization reversal, which suggests a simple and effective method to improve the lifetime of in-vacuum undulators.* PB - JACoW CP - Geneva, Switzerland ER -