Higher-order modes in the bell-shaped single-cell cavity of the SPring-8 storage ring

A single-cell cavity with a bell-shaped inner structure is designed for the SPring-8 storage ring so that the transverse or longitudinal shunt impedances associated with higher-order mode resonances driving coupled-bunch instabilities are low; the highest transverse shunt impedance of the dipole higher-order mode at 1077 MHz is 13.7 MΩ/m and the highest longitudinal shunt impedance of the monopole higher-order mode at 904 MHz is 2.8 MΩ. The shunt impedance of the accelerating TM010-like mode is 6.7 MΩ. The RF characteristics such as resonant frequency, Q-value, shunt impedance and electromagnetic field distribution of the higher-order modes in a model RF cavity were measured.