Empty-bucket techniques for spill-quality improvement at the CERN Super Proton Synchrotron

Synchrotrons can provide long spills of particles by employing resonant extraction where the circulating beam is slowly ejected over thousands to millions of turns by exploiting the amplitude growth caused by a transverse resonance. In the CERN Super Proton Synchrotron (SPS), this method is used to satisfy the experimental requests of the North Area. However, the extracted particle flux is modulated by power-converter ripple, an issue shared across all sychrotrons that perform resonant extraction. In order to suppress such modulations, empty-bucket techniques can be employed, which take advantage of chromaticity to quickly accelerate particles into resonant motion by using a longitudinal rf system. This paper explores empty-bucket techniques via theory, simulation, and measurement, providing a systematic characterization with general applicability to any machine. Additionally, the operational implementation in the SPS is detailed, where the impact on the beam profile and extracted intensity is addressed.