Masaki, M. Dewa, H. Fujita, T. Maesaka, H. Takano, S. JACoW Geneva, Switzerland 978-3-95450-177-9 10.18429/JACoW-IBIC2016-TUPG18 English 361-364 TUPG18 resonance impedance simulation storage-ring vacuum Contribution to a conference proceedings 2017 2017-02 http://dx.doi.org/10.18429/JACoW-IBIC2016-TUPG18 http://jacow.org/ibic2016/papers/tupg18.pdf The requirements for a BPM system for the SPring-8 upgrade are long-term stability, sufficient signal intensity and high accuracy*. The design of a button-type electrode for the BPM has been optimized from the perspectives of 1) mechanical structure, 2) rf characteristics, 3) thermal issue. We have adopted the electrode structure without a sleeve enclosing the button to maximize the button diameter for the narrow aperture of the vacuum chamber. The absence of an annular slot around the sleeve in a lodging hole for the electrode eliminates the associated beam impedance. To minimize the beam impedance and the trapped mode heating of the electrode, the rf structure has been optimized by 3D electro-magnetic simulations. To suppress the ohmic loss on the button and center pin thermally isolated from the water cooled BPM block, we have selected molybdenum as a material with high electric and thermal conductivities. The reduction of the heating suppresses thermal deformation of the electrode and the BPM block, and improves thermal stability of the BPM system. The mechanical tolerance of the electrode was defined to fit the error budget for the total BPM offset error of 0.1 mm rms.