Speaker
Description
Coupled-bunch instabilities restrict the beam intensity in many synchrotrons. These limitations can be mitigated by adding a higher-harmonic radiofrequency (RF) system, which modifies the potential well. Of particular interest is the bunch lengthening mode (BLM), in which the higher-harmonic voltage produces a defocusing effect at the bunch centre. By decreasing the peak line density, space-charge effects are reduced in hadron synchrotrons, while Touschek scattering is mitigated in synchrotron light sources. This contribution investigates the impact of a higher-harmonic RF system in BLM on the intensity threshold, growth rate, and mode behaviour of coupled-bunch instabilities. The influence of beam parameters such as the bucket filling factor, narrowband impedance properties, and RF system configuration is highlighted. Regimes in which BLM is advantageous or disadvantageous are compared to the ones for a single-harmonic RF system. The comparisons are based on semi-analytical solutions of the Vlasov equation as well as macro-particle simulations using the BLonD code.
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