Speaker
Description
As Fixed Field Accelerators (FFAs) are candidates for next-generation, high-intensity
facilities like ISIS-II, characterizing their coherent transverse beam instabilities has become a high priority. Unlike conventional synchrotrons, where head-tail instabilities are managed by manipulating chromaticity, scaling FFAs operate under a strict zero-chromaticity design constraint. This paper investigates how Landau damping from octupolar tune spread can be utilised as a mitigation strategy for the head-tail instability under these zero-chromaticity conditions.
The octupole nonlinearities inherent in FFA magnetic fields induce a substantial tune shift with amplitude. Using PyHEADTAIL macroparticle tracking simulations tailored to the ISIS-II FFA parameters (and its prototype ring the FETS-FFA), we evaluate the growth rate of head-tail modes assuming some realistic impedance source and including this tune spread. We establish the intensity thresholds and transverse impedance limits for stable beam operation, based on these numerical tracking results. The findings demonstrate that intrinsic non-linearities in FFA magnets can provide robust passive stabilization against head-tail modes, validating their viability for future high-intensity applications.