Condition of MA cores in the RF cavities of J-PARC synchrotrons after several years of operation

Nomura, Masahiro  ; Schnase, A.; Shimada, Taihei ; Suzuki, Hiromitsu; Tamura, Fumihiko   ; Yamamoto, Masanobu   ; Ezura, Eiji*; Omori, Chihiro*; Hara, Keigo*; Hasegawa, Katsushi*; Takata, Koji*; Toda, Makoto*; Yoshii, Masahito*

We have been operating the RF cavities loaded with MA cores with a high field gradient of more than 20 kV/m since October 2007. We have been measuring the RF cavity impedance at the shutdown periods, and we detected the impedance reductions of RCS RF cavities on January and June 2009. Taking out the RF cavities from the beam line and opening them, we found that many of cores showed a buckling at the inner radius. We report the detail of condition of MA cores and the relation between the impedance reduction and core condition.