Speaker
Description
Beam feedback systems are crucial in high-energy circular accelerators to suppress undesired bunch oscillations and prevent beam quality degradation. In the longitudinal plane, the CERN Super Proton Synchrotron (SPS) is equipped with beam phase and synchronization loops. The phase loop locks the RF phase of the accelerating voltage to the one of the bunches detected by a pickup, while the synchronization loop maintains the average RF frequency at a programmed reference. During the acceleration ramp, a bunch length excitation leading to significant emittance growth has been observed for different beam types. These longitudinal oscillations vanish when the beam feedback is disabled. A refined simulation model of the SPS beam control loops has been developed for the BLonD simulation suite, allowing to study the impact of longitudinal feedback on single-bunch instabilities. The findings suggest that the instability is intrinsic to the beam due to the radial mode-coupling of the sextupole azimuthal mode, albeit modified by the action of the loops.
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