Excitation of nonlinear second order betatron sidebands for knock-out slow extraction at the third-integer resonance

Radio frequency knock out resonant slow extraction is a standard method for extracting stored particle beams from synchrotrons by transverse excitation. Classically, the beam is excited with an rf field comprising a frequency band around one of the betatron sidebands. This article demonstrates that the third-integer resonance commonly used for the slow extraction induces nonlinear motion, resulting in the appearance of additional sidebands of higher order at multiples of the betatron tune. Measured and simulated beam spectra are presented, revealing these sidebands and the beam’s response to being excited at first and second order sidebands. The feasibility of using a second order sideband for the purpose of slow extraction is demonstrated. This results in a significant improvement in the temporal structure (spill quality) of the extracted beam, but at the cost of higher excitation power requirements. This is observed both experimentally and in tracking simulations. The mechanism behind the observed improvement is explained using beam dynamics simulations.