Beam-based characterization of higher-order-mode driven coupled-bunch instabilities in a fourth-generation storage ring

Longitudinal coupled-bunch instabilities are driven by monopole higher-order modes (HOMs) in the active and passive radio-frequency (rf) cavities at the MAX IV 3 GeV electron storage ring. By combining different beam and rf-based techniques, we have performed a systematic survey of the properties of these resonant modes as a function of cavity temperatures and fundamental mode frequency. This information was then used to setup a HOM model that allowed us to infer optimized temperature ranges for the cavities. An important feature of the optimization method is that it takes into account not only the need to minimize the growth rates (given by the resistive component of the sum of HOM impedances) from each individual cavity but also aims to minimize the total reactive component of the impedance of HOMs that are present in more than one cavity. The resulting small real coherent tune shifts allow the coupled-bunch modes driven by these HOMs to be effectively Landau damped. Further optimization of the cavity temperatures was performed by minimizing the measured electron beam energy spread within the temperature ranges defined by the HOM model. The optimum temperature search was performed at 300 mA stored beam current and in multi-bunch mode using the RCDS (Robust Conjugate Direction Search) algorithm and succeeded in bringing the electron energy spread to within 10% of the zero current value determined solely by quantum excitation and radiation damping.