title: Radiation-induced magnetization reversal causing a large flux loss in undulator permanent magnets.
abstract: 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. Results of numerical and experimental studies carried out to validate the magnetization reversal and quantify the flux loss are presented, together with possible countermeasures against rapid degradation of the undulator performance.
authors.affiliation: JASRI, Koto 1-1-1, Sayo, Hyogo 679-5198, Japan.
authors.forename: Teruhiko
authors.initials: T
authors.lastname: Bizen
authors.affiliation: RIKEN SPring-8 Center, Koto 1-1-1, Sayo, Hyogo 679-5148, Japan.
authors.forename: Ryota
authors.initials: R
authors.lastname: Kinjo
authors.affiliation: RIKEN SPring-8 Center, Koto 1-1-1, Sayo, Hyogo 679-5148, Japan.
authors.forename: Teruaki
authors.initials: T
authors.lastname: Hasegawa
authors.affiliation: JASRI, Koto 1-1-1, Sayo, Hyogo 679-5198, Japan.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Kagamihata
authors.affiliation: RIKEN SPring-8 Center, Koto 1-1-1, Sayo, Hyogo 679-5148, Japan.
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Kida
authors.affiliation: JASRI, Koto 1-1-1, Sayo, Hyogo 679-5198, Japan.
authors.forename: Takamitsu
authors.initials: T
authors.lastname: Seike
authors.affiliation: JASRI, Koto 1-1-1, Sayo, Hyogo 679-5198, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Watanabe
authors.affiliation: RIKEN SPring-8 Center, Koto 1-1-1, Sayo, Hyogo 679-5148, Japan.
authors.forename: Toru
authors.initials: T
authors.lastname: Hara
authors.affiliation: JASRI, Koto 1-1-1, Sayo, Hyogo 679-5198, Japan.
authors.forename: Toshiro
authors.initials: T
authors.lastname: Itoga
authors.affiliation: RIKEN SPring-8 Center, Koto 1-1-1, Sayo, Hyogo 679-5148, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Asano
authors.affiliation: RIKEN SPring-8 Center, Koto 1-1-1, Sayo, Hyogo 679-5148, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Tanaka